Feature Request: Add `Retry-After` header support to http_client

[gena01]

Looks like Retry-After header is not yet honored by http_client. It's a common mechanism to let the client know when to try again once the client hits a service rate limit.

P.S. Looks like input_http_server generates this header as well: https://github.com/benthosdev/benthos/blob/80327853267a033fffed629217af5fa166bc84af/internal/impl/io/input_http_server.go#L369

[Jeffail]

Hey @gena01 sounds like a reasonable addition.