Adds validation to custom fields' default values

[inietov]

Description

When creating/editing models, if we want to add default values to the fieldsets the form doesn't check the validation rules of them. So we can save whatever value we want as default value even when we set the format to the custom field.

This changes tries to fix that and also adds some control dates so it's less probable the user sets the date format incorrectly.

Fixes #11230

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Test Configuration:

• PHP version: 7.4.16
• MySQL version: 8.0.23
• Webserver version: nginx/1.19.8
• OS version: Debian 10

Checklist:

• I have read the Contributing documentation available here: https://snipe-it.readme.io/docs/contributing-overview
• I have formatted this PR according to the project guidelines: https://snipe-it.readme.io/docs/contributing-overview#pull-request-guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes