feat: [VRD-820] Implement Build.date_created

[liuverta]

Impact and Context

Return datetime object as a new Build.date_created attribute.

Risks and Area of Effect

None; entirely new functionality, covered by tests.

Testing

• Unit test
• Deployed to dev env
• Other (explain)

In Python 3.10:

% pytest unit_tests/deployment/test_build.py 
==================================================== test session starts ====================================================
platform darwin -- Python 3.10.6, pytest-7.2.1, pluggy-1.0.0
rootdir: /Users/miliu/Documents/modeldb/client/verta/tests, configfile: pytest.ini
plugins: forked-1.4.0, xdist-3.1.0, typeguard-2.13.3, hypothesis-6.67.1
collected 2 items                                                                                                           

unit_tests/deployment/test_build.py ..                                                                                [100%]

=============================================== 2 passed, 1 warning in 2.67s ================================================

Reverting

• Contains Migration - Do Not Revert

Revert this PR.