R2DBC - Constant name for both jdbc and r2dbc

[rernas35]

We need to declare a constant to reuse in both JDBC and R2DBC driver. This can be updated later.

https://github.com/rernas35/clickhouse-jdbc/blob/41e1afc89c4c5aa6c1e6874148545086deca9d06/clickhouse-r2dbc/src/main/java/com/clickhouse/r2dbc/connection/ClickHouseConnectionMetadata.java#L28