[YAML] Add SplunkIO write SchemaTransform and WriteToSplunk

[schizophrenicmaniac]

Adds a SchemaTransform for SplunkIO writes and exposes it in Beam YAML as WriteToSplunk. It follows the same approach as the Datadog one (#38362). SplunkIO in Java only supports writes, so there is no ReadFromSplunk here.

Each input row becomes a SplunkEvent. event (string) is required. time (int64), host, source, sourcetype, index and fields are optional. fields can be a nested row or a JSON object string, and other columns are ignored. The config covers everything SplunkIO.Write supports (url, token, batch_count, parallelism, disable_certificate_validation, root_ca_certificate_path, enable_batch_logs, enable_gzip_http_compression) plus error_handling. The error output has the same shape as Datadog's: rows that can't be converted come back with failed_row and a 400, and HEC write failures come back with the HTTP status and the payload. Without error_handling, a failed write fails the pipeline.

While testing this I found that SplunkWriteError loses all of its values once it goes through a coder. It has @DefaultSchema(AutoValueSchema.class), but its accessors aren't get-prefixed, so the inferred schema has no fields and every error decodes as null/null/null. SplunkEventWriterTest doesn't catch this because containsInAnyOrder decodes both sides with the same coder. I added a SplunkWriteErrorCoder (same idea as DatadogWriteErrorCoder) and set it on the output of SplunkIO.Write. This changes the output coder of SplunkIO.Write, so I listed it under bugfixes in CHANGES.md.

Changes:

• SplunkWriteSchemaTransformProvider / SplunkWriteSchemaTransformConfiguration, registered in the IO expansion service
• WriteToSplunk in standard_io.yaml and the matching entry in standard_external_transforms.yaml
• SplunkWriteErrorCoder, used by SplunkIO.Write
• Java tests against a mock HEC server, and a YAML integration test with a fake HEC fixture (tests/splunk.yaml)

Fixes #40470

[codecov]

Codecov Report

❌ Patch coverage is 0% with 105 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.75%. Comparing base (76f6e6d) to head (3e1715b).
⚠️ Report is 103 commits behind head on master.

Files with missing lines	Patch %	Lines
...n/apache_beam/yaml/test_utils/splunk_test_utils.py	0.00%	99 Missing ⚠️
sdks/python/apache_beam/yaml/integration_tests.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #40481      +/-   ##
============================================
+ Coverage     56.16%   58.75%   +2.58%     
- Complexity     2288    13917   +11629     
============================================
  Files          1124     2586    +1462     
  Lines        178336   271824   +93488     
  Branches       1489    11243    +9754     
============================================
+ Hits         100170   159708   +59538     
- Misses        75645   106110   +30465     
- Partials       2521     6006    +3485     

Flag	Coverage Δ
python	79.59% <0.00%> (+0.03%)	⬆️

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[github-actions]

Assigning reviewers:

R: @jrmccluskey for label python.
R: @kennknowles for label java.
R: @derrickaw for label yaml.

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