[LH Doc Upload] Form 0781/a

[NB28VT]

Acceptance Criteria

• [x] Update the EVSS::DisabilityCompensationForm::Form0781 job to use the shared endpoint functionality that was implemented in https://github.com/department-of-veterans-affairs/va.gov-team/issues/88297
• [x] Mask this behavior behind a Flipper
• [ ] Add logging
• [ ] Create a document upload record so we can poll Lighthouse later to determine if the upload failed and send a mailer if needed
• [ ] Create a ticket with a plan for a "Canary" style launch and slowly releasing this in production (parent ticket: #90005)
• [ ] Create a "Canary" release dashboard for monitoring this in production (based off of BDD decisions in ticket #91269 )

[AmyPlange]

Blocked waiting for BDD instructions release CC: @ajones446 during standup

[ajones446]

WIP Draft PR: https://github.com/department-of-veterans-affairs/vets-api/pull/18625