Define a struct to share validation rules for PVC specs

[cbandy]

This adds some validation to the PGAdmin data volume spec. Tests show we can simplify these validation rules, which may help keep estimated validation costs low.

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?
  • Have you added automated tests?

Type of Changes:

• Bug fix

What is the current behavior (link to any open issues here)?

A PGAdmin can be created with an empty/invalid data volume spec, causing errors during reconciliation.

What is the new behavior (if this is a feature change)?

• Breaking change (fix or feature that would cause existing functionality to change)

The PGAdmin data volume spec shares the same basic validation we do on other volume specs.