Add Implementation-specific Path Match type

[mlavacca]

Problem Statement

There are currently three types of HTTPRoute path match:

• Exact - core
• PathPrefix (default) - core
• RegularExpression - implementation-specific

The only reason for RegularExpression to be implementation-specific is the lack of a shared standard for regex.
What's missing here, though, is a real implementation-specific possibility such as the one provided by ingress:
https://github.com/kubernetes/kubernetes/blob/83bb5d570580a3f477737fec5c24ba8fc3554264/pkg/apis/networking/types.go#L546-L550.

Use case

At Kong, we have the need for this kind of path matching as the router can use a custom language that we would like to leverage in HTTPRoutes as well. This can be useful for other implementations as well.

[robscott]

@mlavacca What if we supported a domain-prefixed match type (ie konghq.com/foo) that would allow implementation to support arbitrary matches as necessary?

[youngnick]

domain-prefixed type seems like the way to go here, agreed.

[mlavacca]

@mlavacca What if we supported a domain-prefixed match type (ie konghq.com/foo) that would allow implementation to support arbitrary matches as necessary?

Yep, that would certainly work. I like the idea.

[programmer04]

The idea is to be able to support gateway-specific DSL in case something like that exists. For instance, for Kong, it looks like that expressions-language and sometimes its semantics do not fit well into PathPrefix or RegularExpression. It'd a way for power users to achieve whatever they want

[debuggedme]

There are some common requirements such as semver-based routing, which can be matched with regular expressions, but it would be better if custom capabilities were open.

[mlavacca]

/assign