Introduce `UnsupportedProtocol` exception.

[lovelydinosaur]

To be raised for non http/https requests.

Note that this will allow use to drop enforce_http_url in httpx, and instead defer that handling entirely to the transport class.

[lovelydinosaur]

Thought: We also need to handle the no-host-in-URL case.

Currently dealt with here: https://github.com/encode/httpx/blob/9409900898f40ed855e9c7a7664b7e068599a07d/httpx/_utils.py#L268-L282

I guess we need both MissingHostname(...) and UnsupportedProtocol(...), then.

We can drop httpx's InvalidURL after this, and instead mirror these two transport errors.

[lovelydinosaur]

Okay, so I don't think we want a MissingHostname exception class.
Instead what we actually want for handling that case is to expand our ProtocolError, so...

• ProtocolError
  • RemoteProtocolError
  • LocalProtocolError

Everywhere we're currently raising it is a RemoteProtocolError, but we do actually want a small handful of LocalProtocolError cases, including...

• Missing hostname in URL.
• No Host header included. (Eg. that might expose in httpx if you create a invalid Request manually and .send() it.)

Any similar cases.