Add zeek-pqc-migration-observatory package

[sujalavnelavai]

1. Adds zeek-pqc-migration-observatory to the Zeek package index.
2. Package repository is publicly hosted on github.
3. Package provides passive TLS cryptographic-migration observability.
4. Includes cryptographic capability, negotiated key exchange, classification, migration state, and fallback telemetry.
5. Package includes documentation, LICENSE, BTest coverage, and validation evidence.

AI disclosure: AI-assisted tools were used during development of this package for code drafting, documentation, troubleshooting, and review support. The resulting code and documentation were reviewed and validated by the author. Package behavior was tested using the project's BTest regression suite and validation suite, and the final submitted contents are the author's responsibility.

[ckreibich]

Thanks @sujalavnelavai!

[sujalavnelavai]

Thanks @ckreibich for accepting this package.