Add support for precision in DEFAULT NOW

[albertorestifo]

This PR adds supports for passing a precision to the time in the DEFAULT statement, for example:

CREATE TABLE `test` (
    `valid` TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP(3)
)

According to the mysql documentation, CURRENT_TIMESTAMP is an alias for NOW().

This PR also adds a transformation to normalize CURRENT_TIMESTAMP into NOW.

[albertorestifo]

I found another failing case regarding the timestamp precision. I'm working in a fix.

[hrvoj3e]

I think CURRENT_TIMESTAMP should be kept as is - not normalize it.
Reasoning - online MySQL docs offer examples with CURRENT_TIMESTAMP and not NOW (or other aliases).
So this tool would detect schema changes when there are none (because aliasing).
You would essentialy be running an opinionated syntax checker.

E.g. show create table sometablename; returns no value for precision on 10.3.17-MariaDB

`time_created` timestamp NULL DEFAULT current_timestamp(),

Maybe a flag as an opt-in option?

[albertorestifo]

@hrvoj3e I'm willing to drop this change if the maintainers agree with your reasoning, however keep in mind that Schemalex already does normalization.

[hrvoj3e]

however keep in mind that Schemalex already does normalization.

@albertorestifo I did not know that. It makes sense to be consistent across the app - maintainers will decide at the end. :)