Issue #74 - Support microsecond time resolution

[acinep]

Resolves issue #74

• Rename struct fields to clarify microsecond vs. millisecond
• Use ticks to create and parse DateTime objects retaining microseconds
• Use constant value for UnixEpoch
• Update tests to include microseconds
• Removed p/invoke call

The parsing to/from ticks is a little clunky as the netstandard2 and net6 DateTime API don't support microsecond properties. This is available in net7+ which would clean a lot of this up.

Feedback welcome

[acinep]

I added additional test cases with non-zero microseconds. I think its probably fine to use these instead of the existing cases (keeping the total at three per fixture) but I didn't want to remove existing cases without getting the OK to do so.

[coveralls]

Pull Request Test Coverage Report for Build 4612114486

• 12 of 22 (54.55%) changed or added relevant lines in 4 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.0%) to 85.338%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
DuckDB.NET/Utils.cs	4	6	66.67%
DuckDB.NET/DuckDBTimestamp.cs	0	3	0.0%
DuckDB.NET/DuckDBTimeOnly.cs	6	11	54.55%
<!--	Total:	12	22	54.55%	-->

Files with Coverage Reduction	New Missed Lines	%
DuckDB.NET/DuckDBTimestamp.cs	5	46.67%
<!--	Total:	5		-->

Totals
Change from base Build 4609338767:	-1.0%
Covered Lines:	705
Relevant Lines:	824

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💛 - Coveralls