config: fs: introduce fs_config sections

[efiop]

We've introduced things like --config/--remote-config/--fs-config in dvc import/get/ls/[-url], but we are missing a general mechanism to tweak config options by matching urls instead of matching remote section names.

This could be useful for operating on multiple legacy commits, where remote names changed multiple times over the years.

This could also be very useful for in-dvcfile urls (e.g. dvc import-url), where we don't want to use remote:// notation for some reason or we can't use it at all (e.g. in dvc import and chained imports).

Inspired by git's credential sections https://git-scm.com/docs/gitcredentials#_credential_contexts

[credential "https://example.com"]
	username = foo

, we could do something more general like so:

# will match all urls (or more precisely fs instances that we create for those urls) that start with `azure://mycontainer`
[ fs_config "azure://mycontainer1" ]
# could set all config options that one might need, same as with `dvc remote`
connection_string = XXX

Related #9886

Kudos @uditrana