Implement skip_if_absent for the RequestHeaders action

[eguzki]

The Envoys's RequestHeaders action has a field called skip_if_absent that changes the behavior when enabled.

The spec:

• It should be an optional field with a default value: false.
• When skip_if_absent is false, the action behaves as it does now. When the header is absent, the entire descriptor is discarded, no matter what other action of the same configuration tell.
• When skip_if_absent is true and the header is not present in the request, the descriptor is not discarded, however the action does not generate any descriptor entry. This behavior can lead to an empty descriptor. That is allowed. An empty descriptor is still a valid descriptor. Rate limit service can be called with an empty descriptor. Rate limit request with an empty descriptor

limitador_1  | [2023-02-02T11:30:32Z DEBUG limitador_server::envoy_rls::server] Request received: Request { metadata: MetadataMap { headers: {"te": "trailers", "grpc-timeout": "5000m", "content-type": "application/grpc", "x-envoy-internal": "true", "x-forwarded-for": "172.19.0.4", "x-envoy-expected-rq-timeout-ms": "5000"} }, message: RateLimitRequest { domain: "rls-domain-a", descriptors: [RateLimitDescriptor { entries: [], limit: None }], hits_addend: 1 }, extensions: Extensions }

Note the empty descriptor

descriptors: [RateLimitDescriptor { entries: [], limit: None }]

[alexsnaps]

This might not be relevant anymore, given kuadrant-operator#179, where the actions aren't exposed as such anymore