sqltypes: add ToTime conversion funcs

[derekperkins]

there are already conversions for other native types, but datetime conversions were only available in the vitessdriver. This moves that implementation into sqltypes, so it can be more easily accessed elsewhere. I'm specifically interested in using this with VStream.

The actual implementation was done by @vmg in the driver in 2017, so this just moves it into the sqltypes package, adding a similar ToTime method on Value

Checklist

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• [x] Tests were added or are not required
• [x] Did the new or modified tests pass consistently locally and on CI?
• [x] Documentation was added or is not required

[vitess-bot[bot]]

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.04651% with 6 lines in your changes missing coverage. Please review.

Project coverage is 67.31%. Comparing base (f6ca94f) to head (fb6115e). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/sqltypes/value.go	85.36%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #17178 +/- ## ========================================== - Coverage 67.33% 67.31% -0.03% ========================================== Files 1569 1569 Lines 252244 252487 +243 ========================================== + Hits 169858 169961 +103 - Misses 82386 82526 +140 ```

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