feat: add RFC 10005 BGP Link Bandwidth Extended Community support

[digizeph]

RFC 10005 support

Add a typed LinkBandwidth extended-community variant for RFC 10005.

RFC 10005 defines the Link Bandwidth Extended Community as type 0x00 (transitive) or 0x40 (non-transitive), subtype 0x04, with a 2-octet global administrator and a 4-octet IEEE 754 floating-point bandwidth value in bytes per second.

The current extended-community parser retains these encodings only as generic two-octet-AS communities; it does not identify or expose the link-bandwidth value as a typed field.

Acceptance criteria

• Decode both transitive and non-transitive RFC 10005 encodings into a typed variant.
• Expose the global administrator and f32 bandwidth (bytes/second).
• Encode the typed variant while preserving transitivity.
• Add parser and encode round-trip tests, including both transitivity values.
• Add RFC 10005 to the crate-level supported-RFC documentation and regenerate README.md.