* Propagate errors in tls_config instead of unwrap
Ran into `tls_connector` failing and causing our app to panic and shutdown as it seems there wasn't any way to avoid panicking in `tls_config`.
So after talking to @LucioFranco briefly `tls_config` now returns a `Result` instead and propagates errors to the caller, where they can be handled.
* Fix compile warning when tls feature is disabled
This change introduces proper gRPC interceptors that are avilable
regardless of the transport used. Each codegen service now produces an
additional method called `with_interceptor` that accepts a
`Interceptor`.
All examples have been updated to use this new style and interop has a
custom `tower::Service` middleware to echo the headers. There is also a
new `interceptor` example that shows basic usage.
BREAKING CHANGE: removed `interceptor_fn` and `intercep_headers_fn` from `transport` in favor of using `tonic::Interceptor`.
