Add PLACE_HOST_HEADER env var to control PlaceOS client host_header

[viv-4]

Adds a new environment variable, PLACE_HOST_HEADER, and uses it when constructing PlaceOS::Client so staff-api can override the Host header sent to rest-api, etc.

• Some deployments require using a PLACE_URI which is not a configured PlaceOS domain (authority)
• This allows an internal PLACE_URI while defining which PlaceOS domain to use

Previously the header was always taken from the incoming request, which made it difficult to force a different Host in containerised or proxy setups.

[github-actions]

Hey, your PR is titled Add PLACE_HOST_HEADER env var to control PlaceOS client host_header.
This is not a valid conventional commit.
Since we use well-formed PR titles for our changelogs, not fixing this can introduce problems :'(

[stakach]

LGTM