Get and validate invoice given access key in path. The access key is created via the Register receivable endpoint.

Pismo works with a third-party (B3) who registers the invoices.

This endpoint generates 4 possible events:

1. Request to third-party fails - invoice_validation_request_failure-1
2. Request to third-party succeeds - invoice_validation_request_success-1
3. Third-party validation fails- invoice_validation_failure-1
4. Third-party validation succeeds - invoice_validation_success-1