OpenVPN: Port Connection implementation to Zig

[keeshux]

First pass. Port Swift functionality without delving too much into simplifications, except for a few key ones:

• Let daemon looper follow the connection's lifecycle instead of the daemon's lifecycle. This diverges from Swift where the looper lived inside the OpenVPNSession.
• Assume that connections only live within a daemon, never standalone. In other words: always require an explicit serialized_executor, no races as long as it's used to run generic code.
• Omit NetworkMonitor from connection sandbox; reachability and better path events are posted directly.
• Split daemon and connection events for different semantics

Specific to OpenVPN:

• Advertise IV_MTU
• Restore parser parity plus a few fixes/improvements (data ciphers, *-client socket types)

General cleanups:

• Centralize crypto backends and port tests (tests require at least one backend)
• Fold Swift reachability and better path runtime events into a single NetworkMonitor object based on NWPathMonitor
• Make cache profile-scoped
• Reuse stored allocators for deinit
• Link to vendor binaries

Logging in particular will be refactored in another PR.