Add 3.x regex prefix for HTTPRoute

[rainest]

What this PR does / why we need it:

Kong/kong#9022 requires that regular expression paths in Kong routes include a ~ prefix. If we receive an HTTPRoute with a regular expression path (whose canonical Gateway API form is the regex as-is, with no prefix) or some other path type that we convert into a regex (for example, exact match), prepend the prefix.

Which issue this PR fixes:

This addresses the error @randmonkey found in the 3.0 test logs:

2022/08/24 17:02:36 [error] 2142#0: *558 [lua] handler.lua:1153: could not rebuild router via timer: /usr/local/share/lua/5.1/kong/router/atc_compat.lua:417:  --> 1:23
  |
1 | (http.path ^= "/regex-\d{3}-httpbin")
  |                       ^---
  |
  = expected str_char or str_esc, context: ngx.timer

Special notes for your reviewer:

HTTPRoutes have a pretty clear path to handling this: they have a designated regular expression match type, and its canonical form is a regex without our special prefix. If we're using 3.x and we have a regex match, we prepend our prefix when creating the Kong route path.

Ingress is a bit less clear. There's no dedicated regex path type, just ImplementationSpecific. We could read that as requiring that you do provide the path in the exact form the Implementation expects, i.e. that you need to prepend the ~ yourself, but that's not obvious and presents an upgrade path nightmare. If we prepend it for you, it'll break configuration for anyone that reads Gateway docs and does prepend it themselves.

Checking to see if the prefix is present and adding it if not does seem fine though. Regular expressions without the prefix but with ~ as their first actual match character will not work, but these shouldn't occur since that's not a reserved character and should be vanishingly uncommon given that paths usually begin with / (apparently paths that do not are valid per the path-rootless in https://www.rfc-editor.org/rfc/rfc3986#section-3.3, but I don't think those are relevant to HTTP at least). Worst case if you need a regex that begins with ~ you can write your own ~~ to provide both that and the prefix.

Lastly, although we do not know if ImplementationSpecific paths are regexes or not, using the old Kong heuristic seems a valid approach. AFAICT this was a performance optimization on the gateway's side, not something discarded because it wasn't working.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[mflendrich]

I think the cause is Kong/kong#9027, not 9022 (wrong link in the description?)

[rainest]

Closed in favor of #2883 to discard the mountain of force push/test/add label events. Ask me if you want the original branch with separated (if not particularly well-organized) commits.