Consider adding the worker name to the user agent for HTTP replication calls

This issue has been migrated from [#12807](https://github.com/matrix-org/synapse/issues/12807).

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This will help track down which worker exactly is making a request to another worker, such as where traffic on the main process might be coming from exactly.

Alternatives are to grep a bunch of logs to see if a worker can be identified, however this is extra work.