feat: handle SCMP errors on UDP socket and add raw socket

[mlegner]

Originally, I wanted to use the new raw socket as a basis for the UDP socket. However, there are several challenges and the amount of duplicated code turned out to be somewhat limited. So I now kept two separate sockets with some extracted utility functions.

The UDP socket now returns specific errors if SCMP error messages are received. The raw socket can be used to send and receive SCMP info messages (among other custom SCION packets) as shown in the corresponding integration test.

What is still missing (but should be done in a separate PR) is to make some use of the SCMP errors in the PathAwareDatagram; for example, to invalidate cached paths.

Closes #54

[github-actions]

Package	Line Rate	Health
crates/scion-proto/src/scmp	77%	✔
crates/scion/src/pan	78%	✔
crates/scion-proto/src	79%	✔
crates/scion-proto/src/packet	80%	✔
crates/scion/src/pan/path_strategy	83%	✔
crates/scion-proto/src/path	83%	✔
crates/scion/src	80%	✔
crates/scion-proto/src/packet/headers	85%	✔
crates/scion-proto/src/path/standard	92%	✔
crates/scion-proto/src/reliable	95%	✔
crates/scion-proto/src/address	70%	➖
crates/scion/src/daemon	91%	✔
crates/scion-proto/src/path/metadata	100%	✔
crates/scion/src/socket	77%	✔
Summary	79% (1929 / 2427)	✔

[jpcsmith]

Hey Markus, thank's for preparing this, the raw socket. It looks good. My primary concern would be that we could probably add something (trait or enum) that is a ScionPacket and can be used in the send function signature.