Motivation:

GRPCClient can throw an error when run is called more than once or if
run is called after it has been shutdown. Normally this would happen if
a user caller 'run()' more than once, but can also happen if
'withGRPCClient' is called and the client is never used and the body
returns quickly.

Modifications:

- Improve the error message

Result:

Better error message.

Motivation:

Swift 6.1 has been released, we should add it to our CI coverage.

Modifications:

- Add additional Swift 6.1 jobs where appropriate in main.yml,
pull_request.yml
- Add thresholds for Swift 6.1

Result:

Improved test coverage.

`Task.sleep` will by default try and coalesce multiple timers into one,
mostly for client-specific reasons such as performance, power
consumption, etc.

However, this is undesirable on servers, as it can increase latency,
memory usage, and (in the case of gRPC) may result in timeouts not
firing when they should.

We can avoid this by setting the sleep `tolerance` to zero.

Motivation:

We should aim to keep warnings at zero. There were a few littered around
as a result of Swift 6.1.

Modifications:

- Fix the warnings
- Enabled warnings-as-errors in CI to avoid regressions

Result:

No warnings

Motivation:

Currently there's no way to plumb through details from the transport
level to a request handler. Adding this field allows transports, such as
the nio transport, to add the peer certificate to the server context
when using mTLS.

From there there it's easy for an interceptor to take this data and
propogate it forward to a request handler.

Modifications:

This PR adds a single field to the ServerContext that transports can use

Result:

A new field will be accessible to transports and consumers of the API

---------

Co-authored-by: George Barnett <gbarnett@apple.com>

Motivation:

gRPC is moving from specifying the platforms in the package manifest to
annotating code with availability annotations. In order to do this the
generateed code must also be annotated.

Modifications:

- Generate appropriate annotations

Result:

Generated code has availability annotations

---------

Co-authored-by: Gus Cairo <me@gustavocairo.com>