Locality endpoint discovery service (LEDS)

A group of us met this morning to discuss how to move the EGDS issue and PR forward:
- https://github.com/envoyproxy/envoy/issues/9455
- https://github.com/envoyproxy/envoy/pull/10023

The consensus at this point is that we would like to hold on working on and merging EGDS as currently specified and instead develop something that we are tentatively calling IEDS (Individual Endpoint Discovery Service). Note that better name suggestions are appreciated here!

The general idea of IEDS will be to allow a sub-API of EDS to provide individual endpoints within a defined locality/priority. This will avoid quite a bit of complexity around shuffling endpoints within localities/priorities. Additionally, localities/priorities rarely change in a deployment.

The high level flow of the IEDS API will be:
1) Allow the `lb_endpoints` to either be specified inline as today or instead pointed at a config source that configures the IEDS endpoint https://github.com/envoyproxy/envoy/blob/e1962d76c073bb48c57acd0ff2b57390a22394b7/api/envoy/config/endpoint/v3/endpoint_components.proto#L109
2) Determine how the IEDS resource name will be populated. Some proposals have included adding a namespace field. I'm not as clear on this part so I will defer to @htuch and @markdroth on what they want to do here. From my perspective it would be OK to have the config source specify a namespace and then have that namespace passed as the resource name in the discovery request: https://github.com/envoyproxy/envoy/blob/e1962d76c073bb48c57acd0ff2b57390a22394b7/api/envoy/service/discovery/v3/discovery.proto#L42
3) Modify the EDS code to work properly with the sub-API in which we properly do init manager init as well as populate/update the hosts in a locality/priority. This should work correctly with delta/incremental also.

To avoid confusion, note that we are *not* tackling merging/patching as mentioned here: https://github.com/envoyproxy/envoy/issues/8400. That is a much larger project that should not block this effort.

cc @envoyproxy/maintainers @htuch @markdroth @snowp @wgallagher @tomwans @gengleilei @hzxuzhonghu @seflerZ @lambdai @howardjohn @ramaraochavali