Allow custom validation plugin for custom fields

[jakubkrysl]

Environment

• Python version: 3.6.8
• Nautobot version: v1.1.1

Proposed Functionality

Allow to define custom validation via validation plugin for all custom fields.

This might me as easy as adding @extras_features("custom_validators") to CustomField and CustomFieldChoice models.

Use Case

We'd like to enforce format of values for some custom fields, namely cf.asw_owner. We need it to be like team_name <team@email> (manager@email) to make sure changing the value won't break associated integration scripts from other projects. It already did happen, that's why we'd like to implement a check. But it is not possible yet.

Database Changes

None

External Dependencies

None

[jvanderaa]

This specifically needs to take the validation into the Custom Field Choices, above and beyond the validation regex that is available today.

[jvanderaa]

Can assign to myself and Josh S

[jedelman8]

@jvanderaa what's your timing on this? If it is in the next 2-3 weeks, it'll make 1.2, otherwise, 1.3. Can you let us know? :)