Add tower server example

[belak]

Note that this also adds impl From<Status> for http::Response<BoxBody>
in order to make writing middleware which can return an error much
easier.

Motivation

This is based off the example from https://gist.github.com/LucioFranco/9d5cab419797f54fc7b96bc409d8c6e8. It has been expanded into something a little more complex which requires specifying the Request/Response types but allows for intercepting metadata values with an async context. This allows lookups to happen to verify things like auth tokens.

Solution

Most of the code here is the example, but it also replaces Grpc::map_status with an impl From<Status> for http::Response<tonic::body::BoxBody> in order to make it easier to write middleware which can return an error.

[LucioFranco]

Just for now, can we back out the status change and the auth header check and just leave a comment. I'd like to tackle the status change in another PR and think about it a bit more.

[belak]

Just for now, can we back out the status change and the auth header check and just leave a comment. I'd like to tackle the status change in another PR and think about it a bit more.

Done. I've also opened #376 for discussion around adding some method to convert from Status to an http::Response.

[LucioFranco]

Thanks!