Custom field support for device components

[mngan]

Environment

• Python version: 3.8.5
• NetBox version: 2.9.9

Proposed Functionality

Add support for custom fields for Interfaces

Use Case

I would like to record ESI's for LAG bundles and also external id's to link to port objects in our existing DCIM.

Database Changes

Adding CustomFieldModel to the Interface/BaseInterface object hierarchy will result in schema changes, and possibly require refactoring if performance is an issue.

External Dependencies

None

[jeremystretch]

I've tweaked the scope of this to include all device components.

[candlerb]

Link to #1254 #2281 #2510 #2789 #3083