Add flagon support to Audience methods

[maryamsharif]

Adds flagon support to audience methods.

Should be deployed with https://github.com/segmentio/integrations/pull/3021

Testing

Tested by adding flagon to webhooks audience destination and seeing the flagons appear in the call to createAudience.

The family that's accessible to these methods is: centrifuge-destinations.

• Added unit tests for new functionality
• Tested end-to-end using the local server
• [If destination is already live] Tested for backward compatibility of destination. Note: New required fields are a breaking change.
• [Segmenters] Tested in the staging environment
• [Segmenters] [If applicable for this change] Tested for regression with Hadron.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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  Hits            ?    15600           
  Misses          ?     3203           
  Partials        ?     1094           

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[joe-ayoub-segment]

hi @maryamsharif PR deployed