Return to reference type for config by unchecking Sendable conformance

[Jeehut]

I played around with different options, but all ideas I've had to resolve #192 and #201 included breaking changes that require documentation updates and users to update their code. After thinking about it again, I believe we should simply return to a reference type and mark the configuration file as @unchecked Sendable. This effectively turns off the compiler check for Sendable conformance and thread-safety. I think this is fine, firstly because we have our own synchronization mechanism in TelemetryManager when accessing the configuration:

   var configuration: TelemetryManagerConfiguration {
        get { queue.sync(flags: .barrier) { return _configuration } }
        set { queue.sync(flags: .barrier) { _configuration = newValue } }
    }

Secondly, configuration changes are rare, so even if we didn't have any syncing logic, this shouldn't affect users in practical usage as concurrent changes to the configuration is unlikely to happen.

Fixes #192 and fixes #201.