test: spec compliance test validating constants.py against table.csv

[sumanjeet0012]

Closes #41.

What was changed:

• Added multiformats/multicodec as a git submodule in multicodec-spec/.
• Created a new test file tests/test_spec.py that validates CODECS, CODE_TABLE, and NAME_TABLE against the table.csv file from the authoritative upstream spec.
• Updated multicodec/constants.py and multicodec/code_table.py to match the latest upstream table.csv.
• Configured .pre-commit-config.yaml and pyproject.toml to appropriately exclude the submodule from linting.
• Updated the Makefile to exclude the submodule and docs/conf.py (which previously failed due to import location) during make fix.

Why the change was needed:

• The existing codebase lacked a compliance test, meaning the codecs registry could silently drift out of sync with the upstream spec. In fact, running the new test immediately caught missing codecs (mldsa-44-pub etc) that had been added to upstream but not updated in py-multicodec.

How it was verified:

• Validated via make pr, which runs the ruff formatter/linter, mypy typechecks, and the full test suite.
• The new test_spec_table_completeness test passes successfully.