[WIP] Introduce `CompositeHttpHeadersBase` to replace HttpHeadersUtil.merge()

[injae-kim]

related issue: https://github.com/line/armeria/issues/5146

Motivation:

https://github.com/line/armeria/blob/3595dacd9e7675fec309a1029a2a97b829c08682/core/src/main/java/com/linecorp/armeria/internal/common/HttpHeadersUtil.java#L46-L46

• On https://github.com/line/armeria/issues/5146, we found that HttpHeadersUtil.mergeHeaders(..) occupies a large part of CPU cycles due to expensive deep-copy operations

📌 PRs Overview:

1. ✅ (this PR) Introduce CompositeHttpHeadersBase to replace HttpHeadersUtil.mergeHeaders(..)
2. Introduce CompositeHttpBeaderBuilder builder
3. Introduce DefaultCompositeRequest, ResponseHeaders
4. (finally) Replace HttpHeadersUtil.mergeHeaders(..) to Composite*Headers

Modifications:

• Introduce CompositeHttpHeadersBase and CompositeStringMultiMap to replace HttpHeadersUtil.merge()

Result:

• Now we have CompositeHttpHeadersBase so can implement DefaultCompositeRequest, ResponseHeaders

[github-actions[bot]]

🔍 Build Scan® (commit: e9fcd38353a5cbfa133bcc3992ca36cf8feb8eb6)

Job name	Status	Build Scan®
build-windows-latest-jdk-19	✅	https://ge.armeria.dev/s/fagki3cgeavso
build-self-hosted-unsafe-jdk-8	✅	https://ge.armeria.dev/s/7zm72vtzosdem
build-self-hosted-unsafe-jdk-19-snapshot-blockhound	✅	https://ge.armeria.dev/s/ry56m32qmgk56
build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/w5ttm4bkqh7jm
build-self-hosted-unsafe-jdk-17-min-java-11	❌ (failure)	https://ge.armeria.dev/s/qevvxewiy5ptw
build-self-hosted-unsafe-jdk-17-leak	✅	https://ge.armeria.dev/s/cf22vfdohvxbq
build-self-hosted-unsafe-jdk-11	✅	https://ge.armeria.dev/s/cmfbbdk3iuo4e
build-macos-12-jdk-19	✅	https://ge.armeria.dev/s/zkj54c54wbns4

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.47552% with 111 lines in your changes missing coverage. Please review.

Project coverage is 74.21%. Comparing base (75d5af1) to head (e9fcd38). Report is 194 commits behind head on main.

Files	Patch %	Lines
...necorp/armeria/common/CompositeStringMultimap.java	84.67%	52 Missing and 45 partials :warning:
...ecorp/armeria/common/CompositeHttpHeadersBase.java	82.92%	9 Missing and 5 partials :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5340       +/-   ##
===========================================
+ Coverage        0   74.21%   +74.21%     
- Complexity      0    21119    +21119     
===========================================
  Files           0     1803     +1803     
  Lines           0    77283    +77283     
  Branches        0     9948     +9948     
===========================================
+ Hits            0    57354    +57354     
- Misses          0    15254    +15254     
- Partials        0     4675     +4675     

:umbrella: View full report in Codecov by Sentry.
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[injae-kim]

Really sorry for late, I'll update this PR within this weekend 🙇

[injae-kim]

Really sorry for late, I think we can reduce complexity of CompositeHeader by supporting only add with zero-copy.

• If we support zero-copy on add, set, remove, logic is too complex and easy to make bug
  • If we usually don't set, remove after mergeHeaders, we can minimize complexity of CompositeHeader

I found that spring has similar CompositeAttribute impl but it has many bugs.

✅ So, I'll check mergeHeaders's usage and prepare benchmark test too~! 🙇