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

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[codecov]

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 28 commits behind head on main.

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

Additional details and impacted files

@@            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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[vmg]

I don't see why not!