DEBUG-3535 DRY transport Instance and Spec classes

[p-datadog]

What does this PR do?

Follow-up to #4413, this PR moves the common contents of Transport::HTTP::API::Instance and Transport::HTTP::API::Spec classes (which is the one that was typed up in the respective files) to core.

The classes were subsequently augmented with per-component modules. I moved the common code to a base class such that there are still per-component Instance and Spec classes, except now the per-component classes derive from the base class and include the per-component modules.

Motivation:

Implementation of UDS transport for DI

Change log entry
None

Additional Notes:

The Client class is the last remaining duplicated one, it is out of scope for this PR.

How to test the change?
Existing unit tests

[github-actions]

Thank you for updating Change log entry section 👏

^{Visited at: 2025-02-21 06:18:47 UTC}

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-21 06:46:29

Comparing candidate commit ff8f25b in PR branch di-uds with baseline commit 917747b in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 31 metrics, 2 unstable metrics.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.71%. Comparing base (917747b) to head (ff8f25b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4419      +/-   ##
==========================================
- Coverage   97.73%   97.71%   -0.03%     
==========================================
  Files        1365     1363       -2     
  Lines       83337    83309      -28     
  Branches     4219     4217       -2     
==========================================
- Hits        81447    81402      -45     
- Misses       1890     1907      +17     

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: di-uds
Commit report: ff8f25b
Test service: dd-trace-rb

✅ 0 Failed, 20607 Passed, 1371 Skipped, 3m 17.57s Total Time