feat(fetchers): add IETF RFC fetcher

[chaliy]

What changed

Fetch RFC URLs from IETF Datatracker, RFC Editor, IETF, and legacy IETF Tools as canonical RFC Editor plain text.

Why

Generic HTML conversion can alter RFC section layout and ASCII diagrams. The canonical text publication preserves the normative document structure, references, and status boilerplate.

Before / After

Before: RFC pages used generic HTML conversion and varied by source host.

After: Supported RFC URLs normalize to the RFC number and return format: ietf_rfc with canonical plain text, normalized line endings, and UTF-8-safe response limits.

Proof:

cargo test --workspace: passed
cargo clippy --workspace --all-targets -- -D warnings: passed
RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps: passed
cargo build --workspace --exclude fetchkit-python --release: passed

Risk

• Low
• Relies on canonical text publication availability at RFC Editor.
• Internet-Draft URLs intentionally remain handled by generic fetching.

Checklist

• Unit tests are passed
• Smoke tests are passed
• Documentation is updated
• Specs are up to date and not in conflict

Produced by yolop