Draft: Implement PAN struct as main API entry point

[rolbk]

This PR implements the first draft of the new API design proposed in #285.

Changes

Main API Changes

• Merged ASContext and proposed Client into PAN struct: The PAN struct now serves as the main entry point for SCION networking, managing the daemon connection, path pool, and statistics. This struct is meant to be kept around as singleton in the application that uses it.
• No global state: PAN instances are created with New() and can be properly cleaned up, replacing hidden global state
• Cleaner method signatures: No need to pass asCtx to every function - shared state is managed by the PAN instance

Open Questions

• Not sure if it's better to have PAN as a struct or an interface. Current implementation uses a struct.
• Some config options for Dialing and Listening should still be moved to the PAN struct.
• Classes are still somewhat interleaved (e.g., pathRefreshSubscriber, Selector, dialedConn). Further work is needed to cleanly separate concerns.

Example Usage

// Create PAN instance
pan, err := pan.New(ctx)
if err != nil {
    // handle error
}

// Use PAN for dialing
conn, err := pan.DialUDP(ctx, local, remote,
    pan.WithPolicy(policy),
    pan.WithSelector(selector),
)

Related to #285 and #286

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This change is