feat: add datagram trait and path-aware datagram

[jpcsmith]

Depends on #105

This PR

• adds a trait for Datagram oriented sockets that can (in theory) be used by also by a raw socket, and
• adds a socket variant that wraps the aforementioned Datagram sockets and provides path management.

The trait is a WIP and is subject to refinement as we implement more complex path-service varieties. Currently, only an implementation for static paths was added.

Tasks:

• Implement the path-aware socket
• Run the integration tests over the path-aware variant.
• Complete the documentation for the udp_socket module.

[github-actions]

Package	Line Rate	Health
crates/scion-proto/src/address	67%	➖
crates/scion-proto/src	79%	✔
crates/scion/src/daemon	95%	✔
crates/scion-proto/src/packet/headers	86%	✔
crates/scion-proto/src/path/metadata	100%	✔
crates/scion-proto/src/reliable	95%	✔
crates/scion-proto/src/packet	83%	✔
crates/scion/src/pan	49%	❌
crates/scion-proto/src/path	81%	✔
crates/scion/src	79%	✔
Summary	79% (1165 / 1477)	✔

[jpcsmith]

@mlegner I still need to add tests and cleanup the documentation, but would appreciate if if you could already review the design.

[mlegner]

I really like the general setup! 💯

Just a few questions from my side.

[mlegner]

Looks good, just a few nits left.