Add custom URL convertor register

[Kludex]

Custom URL Convertor

Currently, we don't have a way to create URL convertors, meaning that our users can only match the default types. Those are:

starlette/starlette/convertors.py

Lines 75 to 81 in 2b54f42

	CONVERTOR_TYPES = {
	"str": StringConvertor(),
	"path": PathConvertor(),
	"int": IntegerConvertor(),
	"float": FloatConvertor(),
	"uuid": UUIDConvertor(),
	}

To be fair, there's a way: adding a key, value pair on the CONVERTOR_TYPES dictionary. That's not available on Starlette's public API, or documented.

The idea of this PR is to document and introduce a public API that can be used to create custom URL convertors.

• Closes Add user-defined converters to routing #231
• Reattempts Implement custom convertor registration #458

How does Django and Flask handle this?

Both of them have a way to register custom URL convertors. You can find the way Django makes it possible here, and for Flask here.

Other

Recently, there was a PR to introduce time related convertors on #1398, and I think we should follow the path this PR proposes here, and not creating more convertors on Starlette itself.

Missing

• docs (Do we want a single page with this, or it should be a section on the Routing page?)

[Kludex]

I've created multiple convertors here with the purpose of reproducing #1398. I can also simplify the test suite if wanted. Just let me know.

[aminalaee]

I agree with simplifying the tests, as this can be extended for other things like UUID and there's not really much difference. This is really good.

[tomchristie]

Well this is nice and simple isn't it? Seems worthwhile.
Would need some brief docs.

[aminalaee]

@Kludex I guess this PR can close this #1058 with a reference/example, what do you think?

[Kludex]

@Kludex I guess this PR can close this #1058 with a reference/example, what do you think?

Yeah, this PR closes that issue as well. I've used another convertor as an example on the docs, but we can answer that issue with "how to register/change the path convertor now".