[source-bigquery] timestamps are strings

### Connector Name

source-bigquery

### Connector Version

0.4.2

### What step the error happened?

None

### Relevant information

source bigquery emits timestamp columns as plain strings.

it should be changed to emit proper timestamps (so for example, aws datalake destination can partition on them)

### Relevant log output

_No response_

### Contribute

- [ ] Yes, I want to contribute