Make `transport::error::Kind` public, for better programmatic error handling

[BatmanAoD]

Feature Request

Crates

tonic

Motivation

The transport::error::Error type is quite opaque, and not amenable to programmatically distinguishing between different types of errors.

Proposal

There is a Kind enum that is currently pub(crate); I suggest making this pub and adding a public accessor to return the Kind.

If forwards-compatibility with potential future variants is a concern, the non_exhaustive attribute can be applied to the enum.

Alternatives

A set of public functions returning bool could be added to cover each type of error: is_transport_error, is_invalid_uri, etc.