Remove SplunkHecMetrics destination schema and change hist signals endpoint to POST

BreakingBreaking change

This is a breaking change — integrations that rely on the affected behaviour will need updates before the change rolls out.

What changed

Two breaking changes in this diff:

The ObservabilityPipelineSplunkHecMetricsDestination schema (and its sub-schemas ObservabilityPipelineSplunkHecMetricsDestinationCompression and ObservabilityPipelineSplunkHecMetricsDestinationType) have been entirely removed from the spec, and the $ref to this schema has been removed from the ObservabilityPipelineConfigDestination union. Any client configuring a splunk_hec_metrics pipeline destination will no longer have a valid schema definition.
The /api/v2/siem-historical-detections/histsignals/search endpoint has changed from GET to POST. Clients calling this endpoint with GET will receive an error.