Implement Clone for ChainedInterceptor

[c-thiel]

First of all thanks for this great create!

The Problem

Tonic Service Client are designed to be cloned in Multi-Threading/Multiplexing scenarios. There is a section "Multiplexing-Requests" on this in the tonic docs: https://docs.rs/tonic/latest/tonic/transport/struct.Channel.html#multiplexing-requests

As a result a tonic client implements Clone. When using a client build with this crate, a XXXServiceClient<InterceptedService<TonicChannel, ChainedInterceptor>> is returned. Everything except ChainedInterceptor implements Clone.

I need to use the client in a Multiplexing scenario and require the client to implement a lightweight clone to avoid opening a large number of channels.

The Solution

... is not as trivial as I would have though because ServiceAccountInterceptor requires mutability for the token. I changed it to use Interior Mutability instead.

I also had to introduce a new Interceptor trait which I don't like.
The only reason for the new interceptor trait is the ChainedInterceptor chain method which runs calls via the original Interceptor trait on &mut self. This breaks my interior mutability approach though.

I would argue that even if you decide to stick to the old implementation of the ChainedInterceptor , the changed ServiceAccountInterceptor and AccessTokenInterceptor are better off using interior mutability and can still be used in clone-able clients even though we couldn't use the ClientBuilder to obtain them.

PR is coming in a second, let me know if you have any better ideas!

[buehler]

Done via #566