Add support for DateTime64 type.

[tshev]

The type is supported by the DBMS https://clickhouse.tech/docs/en/sql_reference/data_types/datetime64/, but not by this client. This evening I did some initial work (it seemed like an easy task), but I don't know enough about the internals.

[Enmk]

Hi @tshev ! You are welcome to add DateTime64 implementation, on the server side it has same binary layout as Decimal64.

So it should be rather simple since Decimals are already supported by this client.

[tshev]

Thank you, @Enmk. Here we go: https://github.com/ClickHouse/clickhouse-cpp/pull/38/files

[filimonov]

Fixed in #50