Unify the logic of gateway-api supported features

[levikobi]

What would you like to be added:

1. Enrich the documentation of GEP-2162 with an explanation about when a feature is considered supported.
2. (Maybe) Add an exported function to gateway-api, which unifies the logic of supported features. This will improve the adoption of this GEP (as it will make it easier to implement), and will make things more consistent across implementations. Such function will return a list of supported features, given the ConformanceOptions.

Why this is needed:
I don't think it's clear how an implementation should programmatically decide which features it supports.

We have the ExemptFeatures field, which is straightforward - if a feature is in this list, it isn't supported by the implementation.

We also have the SkipTests field. If an extended feature is contained within the features list of the skipped test, then the implementation doesn't support it. (if this isn't the right logic, please correct me).

How can we programmatically decide about the core features?
Let's take HTTPRoute for example. Should an implementation assume that if it has at least one test in the suite, say HTTPRouteBackendRequestHeaderModifier, then it supports HTTPRoute?

This issue came following this discussion

[levikobi]

@robscott @youngnick @LiorLieberman @arkodg, do you have any thoughts regarding this issue?

[arkodg]

@levikobi +1 to adding a util func within this repo to provide
func GetSupportedFeatures(gatewaySuite suite.ConformanceOptions, skippedTests []suite.ConformanceTest) []gwapiv1.SupportedFeature
this allows the improvements in the GEP such as adding hyperlinks for the features to being implemented once in this repo and used by all downstream implementations

reg your comment around HTTPRoute , it represents all core conformance tests/features

gateway-api/pkg/features/features.go

Line 97 in 7cd3c13

var HTTPRouteCoreFeatures = sets.New(

associated with the HTTPRoute resource, so we'll need to modify the getSupportedFeatures func to make sure it doesnt get omitted if any extended features are removed