Supported type conversions in v2

v1 of this driver allowed different types to be passed for a column at insertion type, with this necessary type conversion supported implicitly. Whilst this was convenient, it also allowed the potential loss of precision. Our aspiration for v2 is to support all possible implicit conversions that can be done safely with no precision loss.

Currently, v2 has a number of functional gaps where no precision would be lost and conversion could be safely performed. This issue will be used to track current gaps and the eventual compatibility matrix.

Current support recorded [here](https://github.com/ClickHouse/clickhouse-go/blob/examples/TYPES.md).

Known gaps:

- [x] Insert string to datetime64
- [x] Insert string to datetime
- [x] Insert string to date
- [x] sql.Null* types https://github.com/ClickHouse/clickhouse-go/issues/682