Support naming a virtual channel

[mrmicahcooper]

Allow a GRPC.Channel to be created with a name.

This removes the need for developers to store the Channel that's returned from GRPC.Client.Connection.connect.

This is one attempt to solve the feedback I previously mentioned in a separate issue:
#464 (comment)

I'll explain how I do things now and what this PR is attempting to do:

Current

I have an elixir app that talks to a GRPC endpoint.
So I say {:ok, channel} = GRPC.Stub.connect("https://grpc.endpoint.com")
Now, I need to hold on to this channel somewhere so I can use it later so I create a GenServer with this channel in its state.
Now I call the GenServer which uses the channel in its state to forward the request to the GRPC stub.
Something like HelloWorld.Stub.say_hello(channel, "hello")
This works and has the ability to ensure the GRPC.Client.Supervisor is started before we start this GenServer starts.
The downside is now I have to have an entire process just to store a virtual channel.

Proposed

Use a module attribute to store the name of the channel I want.
@endpoint_channel :my_service_channel
Now create a channel with that name and ignore the response:
{:ok, _channel} = GRPC.Client.Connection.connect("https://grpc.endpoint.com", name: @enpdoint_channel)
When I want to use that channel, I use the predetermined name:
Something like HelloWorld.Stub.say_hello(%Channel{ref: @endpoint_channel}), "hello")
Now I don't need to store that channel anywhere.

This PR implements the "proposed" and ignores the fact that you'd have to manually create this connection somewhere in your app after the GRPC.Client.Supervisor is started.

Something that might be cool is the ability to create these named channels in config.
But this PR seems like a really low risk way add this feature.

I'd love any feedback or thoughts about a different approach.

Thanks!

[sleipnir]

Overall, I thinik this is cool.
But if we're going to do this (expose named channels), maybe it would be better to use a Registry and allow managing multiple channels.
WDYT @polvalente ?

[mrmicahcooper]

Overall, I thinik this is cool. But if we're going to do this (expose named channels), maybe it would be better to use a Registry and allow managing multiple channels. WDYT @polvalente ?

I have no strong opinions on an implementation as you are much closer to the the project than I am. This PR was more to exemplify the desired feature.

[sleipnir]

Thank you @mrmicahcooper!