add blockchain repository

src/sakia/data/entities/__init__.py

src/sakia/data/entities/blockchain.py

src/sakia/data/repositories/__init__.py

src/sakia/data/repositories/blockchains.py

src/sakia/data/repositories/meta.sql

src/sakia/tests/unit/data/test_blockchains_repo.py

src/sakia/tests/unit/data/test_communities_repo.py