Fixed Validation error when creating custom fields' default values

[inietov]

Description

When a user creates custom fields, they can mark such fields as required. Then they can create fieldsets to customize the existent models. Then they can set default values for this custom fields, but when creating those default values we apply a validator so later when creating assets using that model, the default values are valid.

But it can be the case that the user doesn't want to put a default value to every custom field, and if the field is marked as required, then any of the default values can be assigned. This PR eliminates the rule 'required' on fields that have it, so it only apply the format requested by the user.

It doesn't affect the integrity of the system as this changes are at the Asset model level, so when a new asset using this model is actually created, the required rules are still validating the data.

I still have to find a way to not lose every default value when the validator fails, because in the case there's a lot of custom fields it can be tiresome to fill all the data everytime. But it's a little step in making this feature a little more usable.

Fixes freshdesk #30229

Type of change

• Bug fix (non-breaking change which fixes an issue)

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

[snipe]

Looks great, thank you @inietov!