[flags] FL-1115: Support custom endpoints for exposures

[jonathanmos]

What does this PR do?

Connects the custom endpoints configuration property to the request builder for exposure batches.

Motivation

What inspired you to submit this pull request?

Additional Notes

Anything else we should know when reviewing?

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.71%. Comparing base (740290d) to head (7519fdf).

Additional details and impacted files

@@                     Coverage Diff                      @@
##           feature/feature-flagging    #2942      +/-   ##
============================================================
+ Coverage                     70.69%   70.71%   +0.02%     
============================================================
  Files                           834      834              
  Lines                         30378    30380       +2     
  Branches                       5131     5132       +1     
============================================================
+ Hits                          21474    21481       +7     
+ Misses                         7444     7439       -5     
  Partials                       1460     1460              

Files with missing lines	Coverage Δ
...com/datadog/android/flags/internal/FlagsFeature.kt	87.50% <100.00%> (+0.32%)	⬆️
...roid/flags/internal/net/ExposuresRequestFactory.kt	100.00% <100.00%> (ø)

... and 38 files with indirect coverage changes

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[typotter]

LGTM