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ARROW-8581: [C#] Accept and return DateTime from DateXXArray

This PR introduces additions to the public API for `Date32Array` and `Date64Array` by accepting and returning `System.DateTime` in additio to `System.DateTimeOffset`.

The rationale for making this change is explained in detail on the [JIRA ticket](https://issues.apache.org/jira/browse/ARROW-8581), but briefly: making this change avoids a class of bugs that manifest based on the user's timezone, and where it is very easy to unknowingly fall into a trap.

Note that the array builders no longer derive from `PrimitiveArrayBuilder<TFrom, TTo, ...>`: that base class is designed for conversion from exactly one outward-facing type to the underlying type, but these builders now have two outward-facing types.  As a replacement, I have introduced abstract types `DateArrayBuilder` (specific to dates) and `DelegatingArrayBuilder` (more general-purpose).   See inline comments for further details.

Closes #7654 from mr-smidge/arrow-8581/date-array-builder-type-change

Authored-by: Adam Szmigin <adam.szmigin@jetstoneam.com>
Signed-off-by: Eric Erhardt <eric.erhardt@microsoft.com>