Add SecureEnclave stub

[remko]

New API Proposal: SecureEnclave

Motivation:

Swift packages that use CryptoKit's SecureEnclave module (if it is available) cannot compile against Crypto, since Crypto does not define this module. All SecureEnclave operations have to be conditionally compiled in this case.

Since SecureEnclave has runtime detection (SecureEnclave.isAvailable), a stub of the SecureEnclave API that returns false for isAvailable (and throws for all operations) on non-macOS platforms would remove the need for conditional compilation.

One annoyance is that some functions in this API also take LAContext and SecAccessControl parameters. These can be left out, or the structures would have to be stubbed out too.

Importance:

Better compatibility with CryptoKit.