feat(scmp): add enums for error and info messages

[mlegner]

Closes #130

[github-actions]

Package	Line Rate	Health
crates/scion-proto/src/path/metadata	100%	✔
crates/scion/src/daemon	91%	✔
crates/scion-proto/src/packet	80%	✔
crates/scion-proto/src/path	83%	✔
crates/scion-proto/src/reliable	95%	✔
crates/scion-proto/src/address	69%	➖
crates/scion/src/pan/path_strategy	83%	✔
crates/scion/src	80%	✔
crates/scion-proto/src/packet/headers	85%	✔
crates/scion/src/socket	77%	✔
crates/test-utils/src	100%	✔
crates/scion-proto/src/scmp	72%	➖
crates/scion/src/pan	78%	✔
crates/scion-proto/src/path/standard	92%	✔
crates/scion-proto/src	78%	✔
Summary	79% (1940 / 2470)	✔

[jpcsmith]

Hey Markus, looks good.

Suggestion: Move the methods from the Scmp(Informational|Error)MessageTrait traits to be onto the respective enums and variants, and remove the traits. A user can take either an Scmp(Informational|Error)Message, or one of it's variants to their function. Given the enum type, the trait feels unecessary.