Add Sendability annotations on SocketOptionProvider

[Lukasa]

Motivation:

SocketOptionProvider is used to apply a number of socket options. As part of using it in NIOPosix, it became apparent that several necessary sendability annotations are missing. Let's add those.

Modifications:

• Add Sendable constraints with @preconcurrency to SocketOptionProvider.
• Update the BaseSocketChannel conformance to match

Result:

Tighter Sendable constraints on SocketOptionProvider.