Description:

The string match types are a bit messy. They're defined in multiple APIs but look similar. For most EG APIs(Ratelimit and CORS for now, more in the future), the match types are just used to represent the StringMatcher XDS API.

What I'd like to change:

• Create a common StringMatcher that can be used by Ratelimit, CORS, and other APIs that need plain string matching
• Change the name of StringMatcher in Stats to avoid naming conflicting

I would like to raise a PR to refactor it if it makes sense.