Allow for custom request headers

[wthompson-njar]

Description of the Change

Adds support for custom request headers to Request::request() via a new optional parameter $additional_headers = []. This is backwards compatible: existing behavior is unchanged unless a caller passes custom headers. Library-set headers (e.g., Accept, Authorization) take precedence and will overwrite any values for the same keys provided in $additional_headers.

Related Issues: Closes #4 / Closes #17
Related PRs: Supersedes #5 and #16

Benefits

Some integrations require sending extra headers such as:

• Prefer: maxpagesize=...
• Content-Encoding: gzip

Allowing callers to pass headers directly avoids expanding the method signature with new dedicated parameters.

Possible Drawbacks

None identified.

Verification Process

• Manually verified that custom headers are sent and received as expected.
• Confirmed BC by running existing flows with no $additional_headers.
• Deployed and tested in production via a fork without issues.

Applicable Issues

#4 and #17