feat(transport): add user-agent header to client requests.

[alce]

This PR adds a user-agent header to client requests. The default value is tonic/x.x.x. The value can be configured through the Channel builder.

Endpoint::from_static("...").user_agent("Greeter 1.1")?.connect();

The value passed to the user_agent method will be prepended to the default, in this case the full value will be: Greeter 1.1 tonic/0.3.1.

The header is added by a new UserAgent layer in the client's service stack. While this is not the most straight forward implementation, it seemed to be the cleanest and safest one. It also aligns better with other client implementations (grpc-go clients, in particular).

Alternatives

• Remove user-agent from the list of grpc reserved headers.
• Do not filter out the user-agent header in tonic <-> http request conversions.
• Sniff the request after passing it to the interceptor fn to see if user-agent was set, pluck it out and add it manually after mapping the request.
• Move the optional header from the Endpoint to the client::Grpc struct and set the header after converting the request.
• Don't set user-agent headers at all.

...and maybe others.

fixes #453

[LucioFranco]

LGTM, just one style nit, should we consider adding tests for this?

[alce]

Sure, I'll add a test.

[alce]

ha, weird merging of imports, hang on...

[alertedsnake]

This is awesome, @alce - thanks for working on it!

[alce]

@alertedsnake thanks! And thank you for reporting the issue.

[alertedsnake]

Just confirmed this is working perfectly in my real-world situation, so thanks again :)