Conformance and Support matrix enhancements

[youngnick]

This may end up being a GEP, but this issue is to discuss enhancements to the conformance process, with a couple of end goals in mind:

• As a user of Gateway API, I should be able to look at the status of GatewayClass and see information about what features the reconciling implementation supports.
• As a user of Gateway API, I should be able to consult a feature matrix to determine which implementations support which features. These features should be supported by and/or derived from conformance information. (Thanks @candita for the suggestion!)

A related goal is:

• as a Gateway API implementer, the conformance tests I run should be able to autodetect what features are supported and conformance test them automatically.

I've linked these together in this issue because the schema we'll need for the first will end up looking a lot like the column for the second, and that schema will probably be closely related if not identical to what we'll need from the conformance tests too.