RUM-4374: Emulate upload network call for functional tests

[0xnm]

What does this PR do?

Right now we are calling the real intake for the functional tests of Gradle Plugin, and since we don’t have a real API key, we get an error and we expect build failure in our tests. However, it may be some other build failures, so it is handy to emulate network call to make test expectation without any build failures.

This PR adds build parameter dd-emulate-upload-call which can be passed for the build script execution. If it is present, no real network call will be made and we will create a fake response.

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

Attention: Patch coverage is 33.33333% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 80.78%. Comparing base (b1ee483) to head (c3626c4).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #255      +/-   ##
=============================================
- Coverage      81.48%   80.78%   -0.70%     
- Complexity       203      205       +2     
=============================================
  Files             26       26              
  Lines            810      822      +12     
  Branches         104      105       +1     
=============================================
+ Hits             660      664       +4     
- Misses           112      121       +9     
+ Partials          38       37       -1     

Files	Coverage Δ
...tlin/com/datadog/gradle/plugin/DdFileUploadTask.kt	82.54% <80.00%> (+1.09%)	⬆️
...lin/com/datadog/gradle/plugin/internal/Uploader.kt	72.73% <0.00%> (-27.27%)	⬇️
...m/datadog/gradle/plugin/internal/OkHttpUploader.kt	78.20% <12.50%> (-4.34%)	⬇️

... and 1 file with indirect coverage changes

[xgouchet]

Nice job