Add API function to fetch session metadata

[JonasVerhofste]

Consul-template allows fetching the contents of a KV through the key function, but as far as I can tell there is no way to get the metadata from an associated session.
It would be nice to have a function that gets the metadata of a session coupled to a KV entry (e.g. {{ session "my/kv/path" }}) in a way like the service function exposes things.

Useful information in session metadata includes the NodeID of the node holding the session, and service health check ID's. These can be re-used with the node and service functions respectively.

Relevant api docs:
https://developer.hashicorp.com/consul/api-docs/kv
https://developer.hashicorp.com/consul/api-docs/session