Add consistent hashing InputMatcher

[snowp]

Tracks adding a simple InputMatcher that takes a configurable threshold and will consistently hash the input string, matching if the resulting hash value is above the configured threshold:

match = (hash(input) % 100) > threshold

This would allow users to configure a match rule that will consistently match X% of requests based on a random input string (e.g. a tracing header).

[kailun-qin]

@snowp
At which level would you prefer this matching should be added to? Supposing there are actually 3 opts:

1. extending as a separate match logic for SinglePredicate InputMatcher;
2. extending as a new match rule for StringMatcher;
3. adding as a new ConsistentHashMatcher type.

Thanks.

[snowp]

I would add this as a CustomMatcher extension for use within the SinglePredicate. I don't recall off the top of my head whether we've wired up the extension factory logic here, so that might be part of it