expose configuration for envoy's RateLimitedAsResourceExhausted (proj…

…ectcontour#4971)

The Rate Limit filter in Envoy translates a 429 HTTP response code
to UNAVAILABLE as specified in the gRPC mapping document, but Google recommends
translating it to RESOURCE_EXHAUSTED
(see https://github.com/grpc/grpc/blob/master/doc/http-grpc-status-mapping.md)

This commit introduces a new setting to allow contour to forward the same parameter
introduced in envoyproxy/envoy#4879

The default value is disabled to retain the original behaviour of returning UNAVAILABLE,
as changing it would be a breaking change.

Closes projectcontour#4901.

Signed-off-by: Víctor Roldán Betancort <vroldanbet@authzed.com>
Signed-off-by: yy <yang.yang@daocloud.io>

changelogs/unreleased/4971-vroldanbet-small.md

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+expose configuration for envoy's RateLimitedAsResourceExhausted