Requests that modify push rules cannot be routed to workers

[matrixbot]

This issue has been migrated from #13382.

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https://github.com/matrix-org/synapse/pull/5063 allowed worker processes to service read-only push rule-related endpoints. This is an issue to track the remaining push rule-related endpoints that require writes.

The following endpoints cannot yet be routed to worker processes:

• PUT /_matrix/client/v3/pushrules/{scope}/{kind}/{ruleId}
• DELETE /_matrix/client/v3/pushrules/{scope}/{kind}/{ruleId}
• PUT /_matrix/client/v3/pushrules/{scope}/{kind}/{ruleId}/actions
• PUT /_matrix/client/v3/pushrules/{scope}/{kind}/{ruleId}/enabled

I believe this hinges on converting the push rules stream to use a MultiWriterIdGenerator, instead of what we have today? https://github.com/matrix-org/synapse/blob/0d1d3e070886694eff1fa862cd203206b1a63372/synapse/storage/databases/main/push_rule.py#L132-L139

We did a similar piece of work for moving presence off of the main process.

[olmari]

Is this now solved with https://github.com/element-hq/synapse/pull/17037 and https://github.com/element-hq/synapse/pull/17038 ?