[BitSail][Feature]FakeSource supports decimal precision and scale

[kyle-hawk]

Describe the solution you'd like

Subtask of https://github.com/bytedance/bitsail/issues/243

Fake Connector support bigdecimal field, but seems not support specifying precision and scale, like this

    {
      "name": "decimal_type",
      "type": "bigdecimal(9,9)"
    }

this can be useful when we test decimal fields of sink connectors