chore: export config BE handler for transformer, and unstarted http test server

[mihir20]

Blocking rudderlabs/rudder-server#4473

Description

• Export http request handler for BE Config, so that if someone wants to implement its own http client for BE Config for transformer need not to implement their own handler again.
• Export NewUnstartedHttpServer to initialise a server that will be started(start serving request) at a later point in time

Linear Ticket

https://linear.app/rudderstack/issue/PIPE-884/move-backend-config-unavailability-test-to-rudder-server-integration

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

All modified and coverable lines are covered by tests ✅

Project coverage is 67.25%. Comparing base (0c2c23b) to head (f3749c6).

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #374       +/-   ##
===========================================
- Coverage   79.39%   67.25%   -12.15%     
===========================================
  Files          91       12       -79     
  Lines        6819      913     -5906     
===========================================
- Hits         5414      614     -4800     
+ Misses       1130      256      -874     
+ Partials      275       43      -232     

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