[FUNK-1648] Custom Params support for Extensible Webhook Destination

[vaibhav-nanda]

https://segment.atlassian.net/browse/FUNK-1648
Support has been added to send custom params for refresh token request for Extensible Webhook Destination.

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 93.10345% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.56%. Comparing base (8eb8b1d) to head (f3ca3af).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../src/destinations/webhook-extensible/auth-utils.ts	92.30%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2745      +/-   ##
==========================================
- Coverage   79.02%   78.56%   -0.47%     
==========================================
  Files        1089     1037      -52     
  Lines       21565    18821    -2744     
  Branches     4339     3581     -758     
==========================================
- Hits        17041    14786    -2255     
+ Misses       3313     2828     -485     
+ Partials     1211     1207       -4     

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