autosharding: implementation of EndpointMap, autosharding client and LB Policy - #13039

I've added the LB policy that consumes it (AutoShardingLoadBalancer) and the AutoshardingClient it depends on, so every expectation on EndpointMap is now visible at a call site.

Note for updates in EndpointMap:

1. Index is now list position, not a stored field.
2. Index assigned after de-duplication.
3. Rebuild in place instead of a fresh map: every grpc-java LB retains children by key. This is what makes the rebuilding flag necessary: LazyLoadBalancer publishes IDLE synchronously from acceptResolvedAddresses, so without it the LB would build a picker from a half-rebuilt map.
4. The picker snapshot is built here, not in the picker.
5. requestConnection returns early unless IDLE, where the gRFC calls ExitIdle() unconditionally.
6. AutoShardingAttributes.ATTR_ENDPOINT_HOSTNAME and XdsInternalAttributes.ATTR_ADDRESS_NAME (gRFC A81) carry the same value but are two different keys. We need to bridge them when we do xds integration or we can refactor it out in util, we simply cannot let autosharding have a dependency on xds because then it will be cycle when we do xds integration.

I will re-request for your review after I do my final review. Please wait for the re-request @sauravzg before taking a look.