fcds: add unimplemented api

[ohadvano]

Additional Description: Adding unimplemented FCDS API as a first step in roadmap to support this feature. See #39141 for feature description. Most of the specifications are described inline with the API changes but for extra clarity I will describe them here:

• Current FCDS implementation is intended to discover a filter chain object (including its name)
• It's meant to support both delta and SoTW APIs
• A discovery request should include the containing context name. The containing context can either be customized per discovery in the config or derived by the listener's name.
• A listener may contain both statically configured filter chains as well as runtime discovered filter chains. However, if a discovered filter chain matches any of another existing filter chain, its configuration is discarded (this will be later indicated by metrics and/or logs).
• The model here is very similar to how EDS works. A cluster owns an EDS subscription. It uses a custom service name or the cluster name in the discovery request and the server responds with a list of endpoints. In this discovery service, the listener owns the FCDS subscription, mentioned by the listener name or custom configured name, and the server responds with list of filter chains.

Example usage:

static_resources:
  listeners:
  - name: tcp_listener_1
    stat_prefix: tcp_listener
    address:
      socket_address:
        protocol: TCP
        address: 0.0.0.0
        port_value: 1111
    filter_chains:
    - name: some_static_filter_chain
      filters:
      - filter_1
      - filter_2
    fcds_config:
      name: tcp_listener_1_dynamic_filter_chains
      config_source:
        api_config_source:
          api_type: GRPC
          transport_api_version: V3
          grpc_services:
          - envoy_grpc:
              cluster_name: xds_cluster

Risk Level: low
Testing: none
Docs Changes: API
Release Notes: none
Platform Specific Features: none

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @wbpcode
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #39154 was opened by ohadvano.

see: more, trace.

[adisuissa]

Thanks!
cc @markdroth

[ohadvano]

@markdroth, @adisuissa please review latest iteration

[markdroth]

This looks good to me! I'll defer to @wbpcode as the designated API reviewer.

[wbpcode]

LGTM. Thanks.