[PowerShell] Support ApiKeyPrefix

[condorcorde]

This PR contains a small change to support ApiKeyPrefix, i.e. an extra string that may be required in the Authorization: header (or in the URL parameter) before the ApiKey. ApiKeyPrefix is already supported in the Configuration, but never used in the API code. Likely, when required the prefix was added to the key as a workaround.

The prefix string is directly concatenated to the ApiKey. Any space separators that may be required must be added to the ApiKeyPrefix.

The change has been tested on a client API requiring such a prefix in the header.

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@wing328

[wing328]

lgtm. Thanks for the PR

When you've time, can you please PM me via Slack? Thanks