Allow InsertMediaModal to be used with Custom Form Schema

[maxime-rainville]

Story

As a CMS developer, I want the ability to reuse the InsertMediaModal with my own custom form schema so I can build tailored experiences for my content authors.

Acceptance criteria

• Existing InsertMediaModal functionality is retained (ability to upload file, edit existing file, select file, etc.)
• Developers can provide an arbitrary form schema URL and have it rendered in the InsertMediaModal when a file is selected.
• FormSchema actions are honoured.
• Appropriate documentation on how to use the feature is drafted.
• If relevant, opportunities for re-factor of getFormSchema approach are identified and follow up cards are created

Related

• More versatile FileModal maxime-rainville/silverstripe-anyfield#12
• Allow user to edit file information directly in the modal #1038
• Standardise how we create API/FormSchema endpoint in the CMS silverstripe-admin#1589

[maxime-rainville]

We did some good investigation here: #1422

I'm moving this card to the 5.3 milestone and adding it to the AnyField EPIC.