Support HTTP/2 settings in BackendTrafficPolicy for upstreams

[guydc]

Description:

Several HTTP2 Protocol options can have an impact on resilience, performance and security of upstream connections. For example max_concurrent_stream defines "For upstream connections, [...] how many streams Envoy will initiate concurrently on a single connection."

#3222 introduced an API for HTTP2 settings in downstream connections. Envoy Gateway can support the same settings for upstream connections in BackendTrafficPolicy.

Relevant Links:

• https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/core/v3/protocol.proto#config-core-v3-http2protocoloptions