Add support controlling feature availability

[iamdharmesh]

Description of the Change

PR adds an enable/disable feature setting option for the below features. PR also improves the codebase for better maintainability.

• Language Processing / IBM Watson / Natural Language Understanding
• Language Processing / Microsoft Azure / Text to Speech
• Recommended Content / Microsoft Azure / AI Personalizer

Closes #589

Note for the reviewer:
To avoid merge conflict I have started work from enhancement/588 branch and kept it as base branch for this PR.

How to test the Change

1. Go to Classifai settings for the specific feature.
2. Toggle feature availability for the below features and verify that feature availability reflects accordingly on site.

• Language Processing / IBM Watson / Natural Language Understanding
• Language Processing / Microsoft Azure / Text to Speech
• Recommended Content / Microsoft Azure / AI Personalizer

Changelog Entry

Added - Enable/disable toggle option for the "Classify content" feature.
Added - Enable/disable toggle option for the "Text to Speech" feature.
Added - Enable/disable toggle option for the "Recommended content" feature.

Credits

Props @iamdharmesh @jeffpaul

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.