Add support for decimal string format to generator

[Tetraquark]

Now if openapi config has property like this:

amount:
  title: Amount
  type: string
  format: decimal

Then the class with this property will be generated with error:

@SerialName("amount")
val amount: java.math.BigDecimal // can't import java in common code

[Alex009]

will generate BigNum type from https://korlibs.soywiz.com/kbignum/

[Alex009]

will be available in 0.12.0