chore: pass headers to the PHP worker

[rustatian]

Reason for This PR

• PHP Headers skipped in the internal message structure.

Description of Changes

• Add header field to the internal message structure.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.

[codecov-commenter]

Codecov Report

Merging #303 (567e247) into master (942c8b2) will increase coverage by 0.12%.
The diff coverage is 2.02%.

@@            Coverage Diff            @@
##           master    #303      +/-   ##
=========================================
+ Coverage    3.04%   3.17%   +0.12%     
=========================================
  Files          15      18       +3     
  Lines        1575    1605      +30     
=========================================
+ Hits           48      51       +3     
- Misses       1525    1552      +27     
  Partials        2       2              

Impacted Files	Coverage Δ
aggregatedpool/activity.go	0.00% <0.00%> (ø)
aggregatedpool/handler.go	0.00% <0.00%> (ø)
aggregatedpool/interceptor.go	0.00% <0.00%> (ø)
aggregatedpool/workers.go	0.00% <0.00%> (ø)
aggregatedpool/workflow.go	0.00% <0.00%> (ø)
common/context.go	0.00% <0.00%> (ø)
info.go	0.00% <0.00%> (ø)
internal/codec/proto/proto.go	0.00% <0.00%> (ø)
internal/protocol.go	0.00% <0.00%> (ø)
metrics.go	0.00% <0.00%> (ø)
... and 2 more

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