Provide an optional path prefix preference setting for Tokenserver

[data-sync-user]

Per this comment we should create an optional configuration parameter which allows self hosters to add a path prefix to tokenserver and syncstorage URIs.

┆Issue is synchronized with this Jira Task

[kyz]

SyncServer-1.5 let you write the whole URL, like this:

public_url = https://home-server.example.com/prefix

The configuration option should let the user explicitly set the scheme, host, port and path prefix, not just the prefix.

Currently, syncstorage-rs guesses scheme, host and port and can only be configured explicitly by the reverse proxy passing host:port in X-Forwarded-Host and scheme in X-Forwarded-Proto

[kyz]

@manuth has given an example implementation of a public_url configuration option here: https://github.com/mozilla-services/syncstorage-rs/issues/1217#issuecomment-2709133105