Allowed values for Path Prefix Rewrite

[LiorLieberman]

Currently we don't validate values in ReplacePathPrefix.

Users can have the following config:

...
    matches:
    - path:
        type: PathPrefix
        value: /foo
    filters:
        - type: URLRewrite
          urlRewrite:
            path:
              replacePrefixMatch: bar (NOTE the absence of a leading "/")
              type: ReplacePrefixMatch

This will essentially screw the request, (tested with istio, i got 400).

The question is what are our expectations around it and whether we should add CEL to the spec to disallow it.

Technically, for a URI, it is possible to have something like one.exmaple.com#<some-fragment> or one.exmaple.com?<query-params> without a "/" in between, but AFAICT, most browsers normalize that and add a "/" in between, AND, more importantly, this feature is path rewrite and not a rewrite for other parts of the URI.

Thoughts?

/cc @mikemorris @arkodg @robscott @howardjohn @kflynn