Add Ninja Forms conversion tracking.

[benbowler]

Summary

Addresses issue:

• Add Plugin integration for Ninja Forms #8799

Relevant technical choices

Beyond the IB I tracked the event_category using the Ninja forms form_id.

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 7.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.

Merge Reviewer Checklist

• Ensure the PR has the correct target branch.
• Double-check that the PR is okay to be merged.
• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.

[github-actions]

Build files for fdc334d have been deleted.

[github-actions]

Size Change: +729 B (+0.05%)

Total Size: 1.54 MB

Filename	Size	Change
./dist/assets/js/optin-monster-********************.js	673 B	+1 B (+0.15%)
./dist/assets/js/popup-maker-********************.js	634 B	+1 B (+0.16%)
./dist/assets/js/ninja-forms-********************.js	727 B	+727 B (new file)	🆕

ℹ️ View Unchanged

Filename	Size
./dist/assets/css/googlesitekit-admin-css-********************.min.css	54.6 kB
./dist/assets/css/googlesitekit-adminbar-css-********************.min.css	11.2 kB
./dist/assets/css/googlesitekit-authorize-application-css-********************.min.css	770 B
./dist/assets/css/googlesitekit-wp-dashboard-css-********************.min.css	7.47 kB
./dist/assets/js/29-********************.js	2.76 kB
./dist/assets/js/30-********************.js	2.25 kB
./dist/assets/js/31-********************.js	3.64 kB
./dist/assets/js/32-********************.js	935 B
./dist/assets/js/33-********************.js	892 B
./dist/assets/js/34-********************.js	3.11 kB
./dist/assets/js/analytics-advanced-tracking-********************.js	776 B
./dist/assets/js/contact-form-7-********************.js	645 B
./dist/assets/js/googlesitekit-activation-********************.js	24.1 kB
./dist/assets/js/googlesitekit-ad-blocking-recovery-********************.js	59.5 kB
./dist/assets/js/googlesitekit-adminbar-********************.js	34.7 kB
./dist/assets/js/googlesitekit-api-********************.js	10.2 kB
./dist/assets/js/googlesitekit-components-gm2-********************.js	5.87 kB
./dist/assets/js/googlesitekit-components-gm3-********************.js	10.1 kB
./dist/assets/js/googlesitekit-data-********************.js	2.18 kB
./dist/assets/js/googlesitekit-datastore-forms-********************.js	9.13 kB
./dist/assets/js/googlesitekit-datastore-location-********************.js	2.08 kB
./dist/assets/js/googlesitekit-datastore-site-********************.js	20 kB
./dist/assets/js/googlesitekit-datastore-ui-********************.js	10.1 kB
./dist/assets/js/googlesitekit-datastore-user-********************.js	25.2 kB
./dist/assets/js/googlesitekit-entity-dashboard-********************.js	73.9 kB
./dist/assets/js/googlesitekit-i18n-********************.js	3.92 kB
./dist/assets/js/googlesitekit-main-dashboard-********************.js	124 kB
./dist/assets/js/googlesitekit-modules-********************.js	22.2 kB
./dist/assets/js/googlesitekit-modules-ads-********************.js	29.4 kB
./dist/assets/js/googlesitekit-modules-adsense-********************.js	112 kB
./dist/assets/js/googlesitekit-modules-analytics-4-********************.js	138 kB
./dist/assets/js/googlesitekit-modules-pagespeed-insights-********************.js	22.6 kB
./dist/assets/js/googlesitekit-modules-search-console-********************.js	58.5 kB
./dist/assets/js/googlesitekit-modules-tagmanager-********************.js	30.6 kB
./dist/assets/js/googlesitekit-polyfills-********************.js	376 B
./dist/assets/js/googlesitekit-settings-********************.js	61.7 kB
./dist/assets/js/googlesitekit-splash-********************.js	73.3 kB
./dist/assets/js/googlesitekit-user-input-********************.js	48.3 kB
./dist/assets/js/googlesitekit-vendor-********************.js	317 kB
./dist/assets/js/googlesitekit-widgets-********************.js	57.6 kB
./dist/assets/js/googlesitekit-wp-dashboard-********************.js	60.1 kB
./dist/assets/js/mailchimp-********************.js	629 B
./dist/assets/js/runtime-********************.js	1.3 kB
./dist/assets/js/woocommerce-********************.js	652 B
./dist/assets/js/wpforms-********************.js	632 B

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