feat(slo): Add optional pipeline processor in the SLI ingest pipeline for advanced customer

[kdelemme]

resolves https://github.com/elastic/kibana/issues/191737

Summary

This PR adds an optional pipeline processor to the SLO rollup pipeline. If a customer wants to override the rollup data with custom logic (for example maintenance window) they can create a pipeline names slo-{id}@custom and it will be automatically applied. Any error will be discarded.

For example create an SLO and create a custom pipeline containing a Drop processor with that condition (adjusted for the current time in UTC)

def hour = ZonedDateTime.parse(ctx['@timestamp']).getHour(); 
return hour < 9 || hour >= 17

☝🏻 This will drop any document when the hour is not between 9am and 5pm

[obltmachine]

:robot: GitHub comments

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Just comment with: - `/oblt-deploy` : Deploy a Kibana instance using the Observability test environments. - `run` `docs-build` : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[kibana-ci]

:yellow_heart: Build succeeded, but was flaky

• Buildkite Build
• Commit: bd4d61d8824025224eb61e79fc04dae81554a5d6

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Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with: @elasticmachine merge upstream

cc @kdelemme