[PATCH] pgarchives: Add host option for pglister_sync

This patch makes it possible to define a 'host' parameter in order to override the Host header, to be able to properly query localhost.

Context:
I want to set up HTTP auth for a deployment of pgarchives.
In order for pglister_sync to keep working, I need to either authenticate, or disable authentication for localhost.
When using external URL, the request gets routed to apache using the server's external IP, which makes the 'localhost' whitelist fail.
I prefer using the solution over authenticating, as it ensures queries get directly routed to localhost in the process.
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