Add `get_protocols`, `strip_protocols`, and `is_fully_qualified` APIs

I have a use case for the following APIs, and wanted to get opinions on whether they are feasible as additions:

Firstly,

def get_protocols(url: str) -> tuple[str, ...]:
    """Returns all protocols found on a URL."""

-> get_protocols("my-bucket/a.txt") = ()
-> get_protocols("s3://my-bucket/a.txt") = ("s3",)
-> get_protocols("filecache::s3://my-bucket/a.txt") = ("filecache", "s3")

Then,

def strip_protocols(url: str) -> str:
    """Strips all protocols found on a URL."""

-> strip_protocols("s3://my-bucket/a.txt") = "my-bucket/a.txt"
-> strip_protocols("filecache::s3://my-bucket/a.txt") = "my-bucket/a.txt"

And finally,

def is_fully_qualified(url: str) -> bool:
    """Whether a given url has a leading protocol."""

-> is_fully_qualified("my-bucket/a.txt") = False
-> is_fully_qualified("s3://my-bucket/a.txt") = True
-> is_fully_qualified("filecache::s3://my-bucket/a.txt") = True

I believe existing APIs (especially regarding protocol stripping) only strip the first found protocol. The other stuff is more related to the fact that the existing get_protocol returns "file" if no protocol is found, which is not ideal if you want to check whether the given URL is prefixed with an actual scheme.

My use case is that I want to initialize the correct target file system based on a input URI, and supporting cache prefixes already requires number 1) because I also need the target protocol.
Furthermore, I pad input paths with a configurable user prefix, and before initializing the file system, I need to assert that the resulting padded path is a full URI, otherwise the filesystem lookup fails.

Start by locating the existing get_protocol API and the current protocol-stripping behavior. Compare the proposed get_protocols, strip_protocols, and is_fully_qualified examples with existing tests, then clarify the intended semantics and add coverage for unprefixed URLs, single protocols, and nested cache prefixes.