Ingress: Implement Exact and ImplementationSpecific path matches, Pre…

…fix match updated to match spec (#3471)

Since we already had support for regex path matching without checking
the type of the path match, we now have the following rules to ensure backwards compatibility:

- If you specify a prefix path match type, you get a prefix match
  - prefix matches only match path segments, not a string prefix
  - trailing slashes ignored from prefix match string
- If you specify an exact path match type, you get an exact match
- If you specify an implementation specific path match type you get path matching behavior that Contour already supports
  - If it looks like a regex we give a regex match
  - otherwise a string prefix match like Contour already does
  - ImplementationSpecific match is the default if not set when an Ingress
  resource is created so this should give people the default they are
  already using

Fixes #2135

Signed-off-by: Sunjay Bhatia <[email protected]>