Add "merge when checks pass" to available strategies

What does this MR do and why?

This MR adds the service for "merge when checks pass" strategy to available strategies and allows the strategy to be used by frontend by setting auto_merge_strategy param. The strategy availability is behind a feature flag merge_when_checks_pass.

It sets the new strategy backend-wise as preferred over merge_when_pipeline_succeeds, although frontend still sends merge_when_pipeline_succeeds. This will be changed in a follow-up MR.

MR acceptance checklist

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Related to #407352 (closed) and #8128 (closed)