SQLAlchemy backlog

Hi there,

while working on crate/crate-python#391, some backlog items have accumulated. I will gather them within this ticket.

### Internals
We've identified a few shortcomings in the internal implementation of the CrateDB SQLAlchemy dialect. While it seems to work in general, those spots can well be improved, in order to better align with the internal API hooks of SQLAlchemy, and how the CrateDB dialect interacts with that.

- [ ] Migrate away from using the `before_execute` event.
Mike Bayer advised us to hook into `ExecutionContext.pre_exec()` for rewriting `UPDATE` statements instead of using any kinds of engine- or dialect-events, like our current implementation.
Discussions:
- https://github.com/crate/crate-python/pull/391#discussion_r874170754
- https://github.com/sqlalchemy/sqlalchemy/discussions/5915#discussioncomment-2782664
- [ ] Investigate `CompileError: Unconsumed column names`
Issue: crate/sqlalchemy-cratedb#78
- [ ] crate/sqlalchemy-cratedb#84
- [ ] Investigate whether SQLAlchemy's `MutableDict` implementation can be used for implementing CrateDB's `OBJECT` type, see https://github.com/crate/crate-python/pull/561#pullrequestreview-1514116649.

### More
- [ ] Improve code coverage and reduce code duplication of the `visit_update_14` method.
Reference: https://github.com/crate/crate-python/pull/391#pullrequestreview-991961268
- [ ] https://github.com/crate/sqlalchemy-cratedb/issues/79

With kind regards,
Andreas.