C++ Date Validation

[sc1f]

1. Add a date parser on the C++ side that correctly validates date/datetime strings using the iomanip header.

• The parser is not called by any running code, nor is it included in CMakeLists.txt.
• The header does not include support for milliseconds and timezones, which is fine for string validation but will be problematic for parsing.

2. Added tests for the date parser, and moved computed column tests into their own describe block.
3. Fixed the use of now-undefined PSP_ABORT in PSP_VERBOSE_ASSERT2.

[texodus]

Thanks for the PR!