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

## 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.