Support SQLAlchemy dialect with asyncpg

Hi there,

after closing crate/sqlalchemy-cratedb#100 and bringing in crate/crate-python#391, we may want to look at [SQLAlchemy support for Python asyncio]. Currently, trying to invoke the CrateDB driver like `postgresql+asyncpg://` croaks like

```python
>>> from sqlalchemy.ext.asyncio import create_async_engine
>>> create_async_engine("crate+asyncpg://crate@localhost/test")
sqlalchemy.exc.NoSuchModuleError: Can't load plugin: sqlalchemy.dialects:crate.asyncpg
```

So, the CrateDB dialect will have to be accompanied by a special asyncio mediation layer, like [dialects/postgresql/asyncpg.py].

With kind regards,
Andreas.

[SQLAlchemy support for Python asyncio]: https://docs.sqlalchemy.org/en/14/orm/extensions/asyncio.html
[dialects/postgresql/asyncpg.py]: https://github.com/sqlalchemy/sqlalchemy/blob/main/lib/sqlalchemy/dialects/postgresql/asyncpg.py