Remove dead domain codec helpers and tighten certhash decoding

[acul71]

Summary

• remove unused module-level to_bytes/to_string functions from multiaddr.codecs.domain (issue Minor Bug: Dead Code in domain.py Module #100)
• add newsfragments/100.removal.rst documenting the removal
• normalize certhash multibase decode output and enforce u (base64url) prefix to keep typing and protocol validation consistent

Test plan

• source venv/bin/activate && make pr
• source venv/bin/activate && make docs-ci

Closes #100

[acul71]

@yashksaini-coder can you review ?

[acul71]

cast(bytes, ...) was used for type-checking, not runtime behavior.

• multibase.decode(string) is not precisely typed (often Any or a broad type in stubs).
• The code then passes that value to self.validate(...), which expects bytes.
• Static analyzers (mypy/pyrefly) may complain: “this might not be bytes.”
• cast(bytes, x) tells the type checker: “treat x as bytes here.”

Important: cast does not convert anything at runtime.
So this line:

decoded_bytes = cast(bytes, multibase.decode(string))

only silences type errors; it does not protect against decode() returning the wrong shape/type.

That is why your current approach is stronger: it does real runtime normalization/checking and then guarantees bytes before validation.