Add structured parser for Traffic Engineering attribute (RFC 5543, code 24)

[digizeph]

Add a structured parser for the BGP Traffic Engineering attribute (code 24, RFC 5543).

Currently this attribute is raw-retained (AttributeValue::Raw). Full semantic support needs a custom parser because the wire format is a complex fixed+variable GMPLS ISCD descriptor rather than a simple TLV list.

Wire format

Switching Capability (1 octet)
Encoding (1 octet)
Reserved (2 octets)
Max LSP Bandwidth at priority 0-7 (8 × 4 bytes, IEEE float)
Switching Capability specific information (variable)

The content and length of the variable tail depend on the Switching Capability field (PSC-1/2/3/4, etc.).

Proposed scope

• Add a TrafficEngineering model struct with the fixed fields and a raw Bytes field for the switching-capability-specific tail.
• Add parse_traffic_engineering and encode_traffic_engineering functions.
• Wire AttrType::TRAFFIC_ENGINEERING in the parser and encoder (currently raw-retained).
• Add tests for:
  • PSC-1 format with Minimum LSP Bandwidth and Interface MTU
  • unknown switching capability type → retain raw tail
  • malformed/truncated input
  • round-trip encode/decode

Out of scope

• Full GMPLS ISCD semantic decoding (switching-capability-specific TLV parsing).
• Policy validation based on TE data.
• Integration with other TE/GMPLS features.

References

• RFC 5543: https://datatracker.ietf.org/doc/rfc5543/
• RFC 4203 (OSPF ISCD encoding)
• RFC 5307 (IS-IS ISCD encoding)