Add StripeFileUpload model and tests

[joshsmith]

Problem

We'll need a StripeFileUpload that optionally belongs to a StripeConnectAccount (in the db), but has a create_identity_document_changeset/2 that does verify the association.

The model should have a:

• id_from_stripe
• created (in Stripe)
• size
• purpose
• url
• type

See: https://stripe.com/docs/file-upload

[snewcomer]

@joshsmith does this need a controller as well?

[joshsmith]

@snewcomer yes but it can be done as a separate issue, #570. I'd like to do them as separate PRs ideally.

[joshsmith]

Partly this is because #570 is blocked by implementation inside stripity_stripe, which we'll also have to do.

[joshsmith]

See beam-community/stripity-stripe#154 for reference on that.

[snewcomer]

Does this also mean it also has a create_changeset function or just the create_identity_document_changeset/2?

[joshsmith]

Closed by #574