Add `.close` to `Connection`

[erny-powel]

gRPC channels have a .close method that releases resources and terminates active RPCs. We should expose this as Connection.close. In addition we should make Connection a context manager that closes on exit. This mirrors the behavior of channel context managers.

@tnoczyns-volue thoughts?

[tnoczyns-volue]

I like it 👍 Looks like a low hanging fruit from the implementation side. More work would be to update examples and documentation.