Make endpoint types sendable

[porchdog]

Adds Sendability conformance to Endpoint.EndpointType enum. This was mostly mechanical save for BonjourEndpoint and ApplicationServiceEndpoint which needed locking support to protect access to the mutable Storage object.

[agnosticdev]

Thank you!

[rnro]

The way that this pattern (a class-backed-struct) is made Sendable in the NIO family of repositories is to use the CoW as the concurrency protection by avoiding shared mutable state. The struct is then marked as @unchecked Sendable (not ideal but still required I think) e.g. https://github.com/apple/swift-nio/blob/48119dbbd23e3eabba48952ac7f75ebeeb87c217/Sources/NIOCore/ByteBuffer-core.swift#L1483 .

I suspect that we want to take that approach here since we are already using the NIO CoW pattern.