DEBUG-3700 agentless settings resolver

[p-datadog]

What does this PR do?

Adds an agentless settings resolver, which performs the same function as the agent settings resolver to go from settings/Datadog configuration to agent settings, but for agentless operations (currently only telemetry).

Motivation:
Existing code (telemetry transport) did not have the agent settings resolution as an encapsulated concept. As a result, there was low test coverage for this code and for example the timeout was always fixed at 30 seconds.

With the upcoming PR that will change telemetry to use core transport code, telemetry will need to provide agent settings to the transport even in agentless mode. The agentless settings resolver that this PR adds provides the required bridge.

Change log entry
Yes: improve agentless mode telemetry configuration - it now respects timeout specified for the agent

Additional Notes:

This PR shares the with_env helper with #4588. I originally wrote it for this PR and cherry-picked it into 4588.

How to test the change?
A bunch of tests/assertions are added with this PR.

This PR was also tested as part of the in-progress PR for UDS support (#4630).

[github-actions]

Thank you for updating Change log entry section 👏

^{Visited at: 2025-05-08 18:07:25 UTC}

[datadog-datadog-prod-us1]

Datadog Report

Branch report: agentless-resolver
Commit report: fa54149
Test service: dd-trace-rb

✅ 0 Failed, 21239 Passed, 1373 Skipped, 3m 32.8s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-05-08 18:26:53

Comparing candidate commit fa54149 in PR branch agentless-resolver with baseline commit 65cb319 in branch master.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 30 metrics, 2 unstable metrics.

scenario:profiling - hold / resume

• 🟩 throughput [+82287.616op/s; +84654.386op/s] or [+5.064%; +5.210%]

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.85714% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.76%. Comparing base (65cb319) to head (fa54149).
Report is 270 commits behind head on master.

Files with missing lines	Patch %	Lines
.../core/configuration/agentless_settings_resolver.rb	98.33%	1 Missing ⚠️
spec/support/core_helpers.rb	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4590      +/-   ##
==========================================
- Coverage   97.76%   97.76%   -0.01%     
==========================================
  Files        1412     1414       +2     
  Lines       86236    86410     +174     
  Branches     4348     4379      +31     
==========================================
+ Hits        84312    84477     +165     
- Misses       1924     1933       +9     

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