test: add spec compliance validation against multibase.csv

[sumanjeet0012]

Fixes #37

Description

This pull request adds an automated specification compliance test to ensure that py-multibase remains fully compliant with the upstream multibase specification.

By adding the multiformats/multibase spec repository as a Git submodule, we can now automatically validate our manually maintained ENCODINGS array against the authoritative multibase.csv.

Changes Made

• Added https://github.com/multiformats/multibase as a Git submodule located at multibase-spec.
• Updated GitHub Actions CI (.github/workflows/tox.yml) to initialize and checkout submodules via actions/checkout@v4.
• Added tests/test_spec.py, which parsing multibase.csv and validates:
  1. Every encoding currently supported in ENCODINGS (excluding identity) is present in multibase.csv.
  2. The prefix codes exactly match the character specified in the CSV (including multi-byte emoji prefixes like 🚀).
  3. Every status == "final" encoding in the CSV is fully supported by py-multibase.

Verification

• Ran the test suite pytest tests/test_spec.py locally and it executed successfully.
• Code conforms to all project formatting and linting requirements (make lint).