### What?
As described in https://github.com/payloadcms/payload/discussions/10946,
allow passing a custom `collectionPopulationRequestHandler` function to
`subscribe`, which passes it along to `handleMessage` and `mergeData`
### Why?
`mergeData` already supports a custom function for this, that
functionality however isn't exposed.
My use case so far was passing along custom Authorization headers.
### How?
Move the functions type defined in `mergeData` to a dedicated
`CollectionPopulationRequestHandler` type, reuse it across `subscribe`,
`handleMessage` and `mergeData`.
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Co-authored-by: Jacob Fletcher <jacobsfletch@gmail.com>
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