[STRATCONN-5657] - Add batch keys to webhook action destination

[varadarajan-tw]

This PR adds a new hidden batch_keys mapping. This mapping controls how events are batched together. This PR in itself won't have any effect as it depends on https://github.com/segmentio/integrations-go/pull/449. The integrations-go changes are behind a flagon and we'll be doing a controlled rollout.

Testing doc - [ref]

Testing

• 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

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.92%. Comparing base (cf633f8) to head (f6504d3).

Files with missing lines	Patch %	Lines
...ctions/src/destinations/webhook-audiences/index.ts	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2878       +/-   ##
===========================================
+ Coverage   32.57%   77.92%   +45.35%     
===========================================
  Files          14     1047     +1033     
  Lines         703    19261    +18558     
  Branches      119     3750     +3631     
===========================================
+ Hits          229    15010    +14781     
- Misses        474     2927     +2453     
- Partials        0     1324     +1324     

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