Add RTR v2 ASPA PDU parsing/encoding

[digizeph]

It would be useful to add basic RTR v2 ASPA PDU parsing/encoding support.

This is separate from the BGP path-attribute work: ASPA is not a BGP path attribute, it is RPKI/RTR data that can later be used for AS_PATH verification.

Initial scope:

• add RTR protocol version 2
• add ASPA PDU type 11
• parse/encode the ASPA PDU fields:
  • flags
  • AFI flags
  • provider AS count
  • customer ASN
  • provider ASNs
• add basic round-trip and malformed-length tests

Out of scope for this issue:

• ASPA AS_PATH verification logic
• parsing ASPA RPKI/CMS objects directly

Relevant drafts:

• ASPA profile: https://datatracker.ietf.org/doc/draft-ietf-sidrops-aspa-profile/
• ASPA verification: https://datatracker.ietf.org/doc/draft-ietf-sidrops-aspa-verification/
• RTR / 8210bis work: https://datatracker.ietf.org/doc/draft-ietf-sidrops-8210bis/

Success criteria

The implementation is done when:

• RtrProtocolVersion supports RTR v2.
• RtrPduType supports ASPA PDU type 11.
• There is a public model for ASPA PDUs that exposes at least:
  • protocol version
  • flags
  • AFI flags
  • customer ASN
  • provider ASNs
• The RTR parser can parse ASPA PDUs from bytes without panicking.
• The RTR encoder can serialize ASPA PDUs back to bytes.
• Round-trip tests pass for:
  • ASPA announcement with one provider ASN
  • ASPA announcement with multiple provider ASNs
  • ASPA withdrawal/delete with zero provider ASNs
• Malformed input tests pass for:
  • PDU length too short
  • provider AS count inconsistent with payload length
  • unsupported RTR version / unsupported PDU handling remains correct
• Existing RTR v0/v1 tests still pass unchanged.
• Public docs/comments clearly state that ASPA support is draft-based and does not implement AS_PATH verification.
• The full test suite passes with:

cargo test --all-features

Optional but nice:

• Add a small example or doc test showing how to inspect an ASPA PDU.
• Add changelog entry noting RTR v2 ASPA PDU parsing/encoding support.