feat: add mock config BE support for transformer

[mihir20]

Description

Add support to mock config backend with transformer resource initialisation. This will allow tests to run independent of actual config backend. Test will not fail even if control plane is down.

Linear Ticket

https://linear.app/rudderstack/issue/PIPE-804/mock-config-be-for-transformer-resource-in-go-kit

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Attention: Patch coverage is 78.94737% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 34.39%. Comparing base (fba3feb) to head (f8d5139).

Files	Patch %	Lines
...resource/transformer/transformer_backend_config.go	70.96%	7 Missing and 2 partials ⚠️
testhelper/httptest/httptest.go	72.00%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #344       +/-   ##
===========================================
- Coverage   79.53%   34.39%   -45.15%     
===========================================
  Files          86       47       -39     
  Lines        6627     3815     -2812     
===========================================
- Hits         5271     1312     -3959     
- Misses       1091     2380     +1289     
+ Partials      265      123      -142     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.