[C++] Implement casts from date types to Timestamp

[asfimport]

Discovered the absence of this while refactoring cast.cc. Since we can cast Timestamp -> date, we should be able to cast the other way

Reporter: Wes McKinney / @wesm
Assignee: Ben Kietzman / @bkietz
Watchers: Rok Mihevc / @rok

Related issues:

• [C++] C++ array kernels framework and execution buildout (umbrella issue) (is a child of)
• [C++][Dataset] Minimize Expression to a wrapper around compute::Function (is fixed by)

_{Note: This issue was originally created as ARROW-8876. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
This will be resolved by ARROW-10322