feat: [datastream] Add precision and scale to MysqlColumn (#9677)

* feat: Add precision and scale to MysqlColumn

- Precision and scale are now supported for MySQL columns.

PiperOrigin-RevId: 550322658

Source-Link: googleapis/googleapis@07457fc

Source-Link: https://github.com/googleapis/googleapis-gen/commit/cbb318f145db680fbc382ba02c20de7a47b594fc
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