Add excon instrumentation for AppSec

[y9v]

What does this PR do?
This PR adds SSRF detection to excon http client library.

To enable excon instrumentation:

Datadog.configure do |c|
  c.tracing.enabled = true
  c.tracing.instrument :rack

  c.appsec.enabled = true
  c.appsec.instrument :excon
end

Motivation:
We want to enable SSRF detection for most popular ruby http client libraries.

Change log entry
Yes. AppSec: Add detection of Server-Side Request Forgery attacks for excon http client.

Additional Notes:
Auto-patching for excon is set to false, since it might be used as an adapter for faraday http client, which has auto-patching enabled.

How to test the change?
CI and app-generator (rails-ssrf application)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-18 17:06:26

Comparing candidate commit 7b6d2c9 in PR branch appsec-add-excon-instrumentation with baseline commit 5e20f11 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 33 metrics, 2 unstable metrics.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.96970% with 4 lines in your changes missing coverage. Please review.

Project coverage is 97.69%. Comparing base (5e20f11) to head (7b6d2c9).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...adog/appsec/contrib/integration/excon_ssrf_spec.rb	95.45%	2 Missing ⚠️
lib/datadog/appsec/contrib/excon/integration.rb	95.00%	1 Missing ⚠️
lib/datadog/appsec/contrib/excon/patcher.rb	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4399      +/-   ##
==========================================
- Coverage   97.71%   97.69%   -0.03%     
==========================================
  Files        1361     1366       +5     
  Lines       83241    83372     +131     
  Branches     4227     4230       +3     
==========================================
+ Hits        81339    81450     +111     
- Misses       1902     1922      +20     

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: appsec-add-excon-instrumentation
Commit report: 7b6d2c9
Test service: dd-trace-rb

✅ 0 Failed, 20768 Passed, 1372 Skipped, 3m 13.47s Total Time

[Strech]

I have 2 minor comments, well done 👏🏼