Support for `field.types` in `dbWriteTable()`

[nbenn]

Currently only NULL is accepted as value for field.types in dbWriteTable(). How could this be relaxed?

• Can we forward schema information as SQL to the adbc backend?
• Or could we specify column types in terms of arrow types? Would this be DBI compliant?
• Do we have to circumvent adbc bulk ingestion whenever field.types is not NULL?