add From<i32> impl for GrpcCode

[gshipilov]

This change adds a From<i32> impl for the GrpcCode enum. This allows users to easily convert the result of the GrpcErrorsAsFailures back into an enum.

Fixes: #505

Motivation

I recently had to implement a custom TraceLayer on-failure struct, and it made sense to reuse the GrpcErrorsAsFailures class to extract the status. Unfortunately, it returns the error code as a NoneZeroI32 but tower_http doesn't have a convenient way to map this back to the num.

Implementing this is trivial, but still feels like a reasonable addition to this library.

Solution

I feel like this solution is pretty self-explanatory :).

[jlizen]

This seems fine to me! It matches tonic's approach:

• https://github.com/hyperium/tonic/blob/3ab377b881b068a45995abab622f86b24588d81d/tonic/src/status.rs#L849-L871

(including the lossy fallback to Unknown)

Let me throw in a quick test and comment, and get this merged.