Add own enumeration of data types to API

[katzyn]

In JDBC 4.2 integer constants from java.sql.Types have better alternative: a java.sql.SQLType interface. There is a JDBCType implementation that is equivalent to SQLType and each database can have an own implementation with own types.

I think we need to create such implementation (possibly an enumeration) in org.h2.api package and list all own data types (including intervals, geometries, JSON, etc.) in it.

[grandinj]

sounds good

[katzyn]

Such vendor-specific types have textual names, but they still need some integer constants. I don't want to use constants from Value for that purpose, because they historically have strange order, for example, datetime values with time zones are far from other datetime data types. They aren't required to be sequential, any integer values may be used, we may leave gaps between each category of values for possible expansion in the future.

In the source code names and numbers aren't normally used, because there are no lookup methods for them. If we create an own enumeration, its constants will be used as is and it doesn't really matter how getName() and getVendorTypeNumber() will be implemented. But it's better to return some fixed numbers that aren't going to be changes in the future releases of H2.

[katzyn]

Oracle's driver uses Integer.MIN_VALUE for all strange data types.