Contrib: Add a few SQLAlchemy patches and polyfills

[amotl]

About

Recent work on unlocking MLflow and LangChain to work together with CrateDB yielded a few patches and polyfills needed to make this happen. A few have been upstreamed into the SQLAlchemy dialect and shipped with the crate-python package already, a few others are being added here now, because they do not fit well into the vanilla Python driver / SQLAlchemy dialect.

References

• Add adapter wrapper for MLflow/CrateDB, based on monkeypatching mlflow-cratedb#5
• Add support for CrateDB to LangChain LLM framework crate-workbench/langchain#1
• SQLAlchemy: Improve DDL compiler to ignore unsupported UNIQUE and FOREIGN KEY constraints crate-python#582
• Improve exception handling: Properly raise IntegrityError exceptions crate-python#583
• SQLAlchemy: Ignore SQL's "FOR UPDATE" clause crate-python#584

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