Add support for query parameters to the prometheus remote_write source

[bruceg]

Ref: #5144 (comment)

We've started adding config to other HTTP-based sources to optionally assign query parameters as fields on the events:

https://github.com/timberio/vector/blob/73a71363ea9115843f503801cac1f76bac752f19/src/sources/logplex.rs#L55-L65

I could see extending that to here. It can be useful to allow custom labelling by users by adding query parameters to the endpoint that points to the remote write source.

I could also see supporting setting of HTTP headers as fields similar to the vanilla HTTP source. It might be nice to support those features uniformly for HTTP-based sources.