Add support for Decimal256

[victor-sushko]

Decimal256(S) is an alias for Decimal(76, S). Currently all values of type Decimal(P, S) are converted to decimal and there is no intention to change it.

But Decimal256 and Decimal128 can store values which are bigger than the max value of decimal. It's not clear which .NET type could be used for big decimal values. Maybe it should be some custom type.

[MaceWindu]

Usually providers implement custom type when there is no suitable .net type available, e.g.

• SqlDecimal for sql server/ sql ce
• DB2Decimal and DB2DecimalFloat for DB2
• FbDecFloat in Firebird
• IfxDecimal for Informix
• MySqlDecimal for MySql
• OracleDecimal for Oracle

as alternate/additional option it makes sense to support string type for decimals