GEP: Configurable Retries in HTTPRoute

[robscott]

What would you like to be added: I would like to be able to configure the following in HTTPRoute:

1. The max number of times to retry a request
2. The reason(s) and/or status codes a request should be retried
3. The timeout for each retry attempt

I believe all 3 of these would be implementable for Envoy based implementations, the first 2 would be implementable for HAProxy based implementations, unclear what would be implementable for NGINX or others (cc @pleshakov @shaneutt).

Why this is needed: This is a common feature request and represents a concept that would likely get tied to a variety of custom policies if we did not include it in the main API.