Adoption of Swift 6 features

[rebello95]

I'm opening an issue to track some low-hanging fruit with regard to Swift 6 features that we should explore in Connect-Swift:

• [ ] Enforcing strict concurrency: I believe all these warnings were fixed with https://github.com/connectrpc/connect-swift/pull/309, but we should switch the toolchain to Swift 6 to enforce this
• [ ] Typed throws: Would it make sense to add throws(ConnectError) in some places?
• [ ] Utilize package visibility: The files in this directory are commented as internal-only but are public so they can be consumed by ConnectNIO. We can make these package instead once running on the updated toolchain to remove these from the public interface

We are currently blocked on fully adopting Swift 6 by https://github.com/apple/swift-protobuf/issues/1729.

[rebello95]

https://github.com/connectrpc/connect-swift/pull/331 will allow us to compile with Swift 6