Add from/load/to/save <uri/file>

[eliaskosunen]

Supersedes #3500, related to https://github.com/tenzir/issues/issues/328

In this PR:

• OP <uri> is mapped to OP <uri-scheme> <rest-of-uri>, where OP is one of (from, load, to, save), and <uri> is a RFC 3986 -compliant URI.
• OP <something-else> is mapped to OP file <something-else> if <something-else> isn't a possible plugin name (doesn't conform to the pattern [A-Za-z0-9-_]*)

This effectively means, that the following mappings are implemented:

• from https://example.com/foo -> from https example.com/foo
• from gs://foo/bar -> from gcs gs://foo/bar
• from file:///tmp/file -> from file /tmp/file
• from /tmp/file -> from file /tmp/file
• from ./local-file -> from file ./local-file
• from local-file -> error (no loader local-file found)
• from https -> error (no URL given to https loader)

The logic implemented in this PR is as follows:

This current implementation accepts from https:foo.bar/baz as from https foo.bar/baz. It's strictly not RFC 3986 -compliant, but I don't think it's that big of an issue.

• Implement from <uri> [options...] [read <parser>]
• Implement from <path> for from file <path>
• Same for load
• Same for to and save
• Consider some loader restrictions + remapping (e.g., for gcs)

[jachris]

Sorry that the review took so long. Had too many things to do, but it's very nice to get this feature finally!

[jachris]

The improved UX is very nice! Thanks.

[jachris]

Unless there are any remaining issues, I think we can merge this 🚀

cc @dominiklohmann @mavam

[dominiklohmann]

I think we can merge this, yes.