[BUGFIX] Allow LocalDriver baseUri to be set to any value

LocalDriver evaluates a configuration option baseUri,
to use this URI instead of a derived one from the absolute
base path of files.

However there are currently limitations that need to be fixed
to allow more complex setups:

1. baseUri is not defined in flexform, thus cannot be set from UI
2. baseUri isn't evaluated, when the absolute file path
   is within TYPO3's public path
3. baseUri is currently required to be a fully qualified URI (with scheme and host)

This change addresses all three of them to cover the following use cases:

1. Provide a different hostname to serve the files from, even
   when they are located on the same machine.
2. Allow base URI without host name, for files that are on the same host,
   but in a folder not within TYPO3's root directory (e.g. due to special
   web server configuration)

Resolves: #91232
Releases: master, 10.4
Change-Id: I7694661f9892c612489c68ef271f965df65744b6
Reviewed-on: https://review.typo3.org/c/P...