Add option to restrict datetime precision

[simolus3]

Extracting sub-millisecond accuracy out of MySQL will require powersync-ja/powersync-mysql-zongji#15, so this is WIP.

This adds the timestamp_max_precision option for the config block when defining sync rules. Valid options are seconds, milliseconds and microseconds. Restricting precision is convenient in some cases, e.g. when other parts of an app can't handle sub-millisecond precision (something we noticed for some users migrating to edition: 2 which pads postgres timestamps to six digits).

When a source database provides a time value, it now also provides:

• The inner precision of that time value (milliseconds for mongo, micros for mysql and postgres). That doesn't mean that every value has that many fractional digits (postgres could e.g. emit 11:03:00.000000 as 11:03:00), but it reflects the maximum precision supported.
• A default precision to use when no max precision is configured on sync rules. This matches the inner precision for mongo and postgres. For mysql, we use milliseconds as the default precision for backwards compatibility, while supporting microseconds as an opt-in.

Depending on compatibility options, a time value is then formatted with a precision of min(timestamp_max_precision ?? source.default_precision, source.inner_precision). We also pad values to ensure we consistently use the same amount of digits.

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