feat: Allow custom headers

[typotter]

This replaces #3604

Motivation

We are allowing the SDK to proxy its configuration and as such, need to be able to set request headers.

Changes

• add customHeaders to the provider init options
• add overwriteHeaders option to suppress dd-application-id and dd-client-token from being included in the request

Test instructions

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 91.98%. Comparing base (e0c1a52) to head (31f1d58).

Files with missing lines	Patch %	Lines
packages/flagging/src/openfeature/provider.ts	0.00%	8 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           flagging-dev    #3660      +/-   ##
================================================
- Coverage         92.07%   91.98%   -0.10%     
================================================
  Files               326      326              
  Lines              8201     8206       +5     
  Branches           1856     1860       +4     
================================================
- Hits               7551     7548       -3     
- Misses              650      658       +8     

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[typotter]

/merge

[dd-devflow]

View all feedbacks in Devflow UI.

2025-06-30 03:50:53 UTC ℹ️ Start processing command /merge

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2025-06-30 03:50:57 UTC ℹ️ MergeQueue: pull request added to the queue

The expected merge time in flagging-dev is approximately 0s (p90).

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2025-06-30 03:53:36 UTC ❌ MergeQueue: The build pipeline contains failing jobs for this merge request

Build pipeline has failing jobs for b3722e6:

• check-staging-merge

⚠️ Do NOT retry failed jobs directly (why?).

What to do next?

• Investigate the failures and when ready, re-add your pull request to the queue!
• If your PR checks are green, try to rebase/merge. It might be because the CI run is a bit old.
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Details

Since those jobs are not marked as being allowed to fail, the pipeline will most likely fail.
Therefore, and to allow other builds to be processed, this merge request has been rejected and the pipeline got canceled.