HTTPRoute matching should support custom match types

[youngnick]

What would you like to be added:

See the discussion here for more detail (#822 (comment)), but in short:

In GEP-820 (#820 and #822) we are removing the extensionRef in the HTTPRoute matches section. This issue covers work to add something back in that allows implementations to try custom matching of some sort.

@bowei suggested:
Modify HTTPPathMatch.Type:

• Allow qualified names (e.g. acme.io/my-custom-match). These represent custom match types
• The standard match types (Exact, Prefix, RegularExpression) stay
• Remove ImplementationSpecific

Why this is needed:
We agreed to go ahead with #822 on the proviso we come back, this is to make sure this doesn't get lost.

[robscott]

This was also suggested by @danwinship in #780 (comment).

[hbagdi]

While I understand the motivation here, I think we should refrain from adding this. Here is why:

• Prefix, Exact, Regex, ImplementationSpecific are 4 types. That's enough of a burden we are putting on our users.
• From a product view, being clever with matches is asking for trouble. I frequently advise users to stay away from fine-grained matching semantics because mistakes are easily made and the blast radius is often larger than one can anticipate. Use another dumber matching criteria when possible.
• IME, asking the user to further select fine-grained differences at this level of detail doesn't go well. It becomes really hard to communicate the differences and the overall UX suffers quite a bit. At Kong, we introduced one such field and we failed. Even the maintainers have a hard time distinguishing the differences.
• Although on the surface it may feel wrong to be more opinionated and be restrictive, I think opting for simplicity and clarity is better in this case. Optimizing for esoteric cases at the cost of harming UX for 99% of the users rarely (if ever) gives any positive returns.
• AFAIK, we do not even have use-cases for this. There should be very good reasons to justify adding this.

[hbagdi]

/assign