service topologies: Add node locality token support to proxy

[mateiidavid]

Link to RFC

Problem Statement

In order to make use of service topology, destination svc needs to calculate the priority of an endpoint.

Proxies should have the name of the source node accessible as an environment variable. Through this change, the proxy should take the environment variable value (i.e LINKERD2_PROXY_SOURCE_NODE) and send it along with the service FQDN/IP as part of the Destination.Get call, when querying for the endpoints of a service through the destination svc.

This change will allow the destination svc to infer the topological domain of the source pod.

Acceptance Criteria

• The proxy needs to read the value of the LINKERD2_PROXY_SOURCE_NODE environment variable
• The proxy needs to have a new source_node field as part of the dst::Config struct
• The value of LINKERD2_PROXY_SOURCE_NODE needs to be stored in the new source_node field
• The source_node field needs to be sent along with the rest of the payload when a new call to the discovery service is made

[mateiidavid]

Closed by #4771