Align README and meta.json dependency declarations

[seanwevans]

Motivation

• Make dependency declarations authoritative and consistent between README.md and meta.json so installers and package registries match runtime behavior.
• Explicitly surface which dependencies are required for extension installation versus which are only needed for optional features (HTTPS helpers).
• Ensure install instructions in the README reflect the runtime prerequisites so CREATE EXTENSION succeeds when prerequisites are present.

Description

• Updated README.md to provide a single authoritative dependency list split into Required (extension install/runtime) and Optional (feature usage), and fixed a formatting issue in the Advanced Operations list.
• Added a short explanatory note in README.md stating that plpython3u is required at extension creation because HTTPS helper functions are defined with LANGUAGE plpython3u, while only HTTPS features invoke those helpers.
• Updated meta.json prereqs.runtime.requires to include plpython3u alongside PostgreSQL, plpgsql, pgcrypto, and pg_trgm.
• Added an x_feature_dependencies entry in meta.json documenting that the https_transport feature uses plpython3u and why it is required at install time.

Testing

• Verified meta.json remains valid JSON with python -m json.tool meta.json, which succeeded.

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Codex Task