Add 0781 lighthouse flippers to API provider factory

[ajones446]

This is the first step in allowing us to progressively transition uploading 0781 forms to the new Lighthouse Documents Benefits API.

All new behavior is hidden behind a killswitch flipper, disability_compensation_use_api_provider_for_0781_uploads, which will only use the API provider if turned on.

If the killswitch is enabled, the API Provider Factory itself will decide if we use the Lighthouse or EVSS provider client for the uploads, based on the status of the disability_compensation_upload_0781_to_lighthouse flipper.

Summary

• This work is behind a feature toggle (flipper): YES (see above)
• (Which team do you work for, does your team own the maintenance of this component?) Benefits Disability
• (If introducing a flipper, what is the success criteria being targeted?) 100 percent of all uploads to Lighthouse succeed

Related issue(s)

• Link to ticket created in va.gov-team repo OR screenshot of Jira ticket if your team uses Jira
  ([LH Doc Upload] 0781/a (1) implementation va.gov-team#83846)

Testing done

• New code is covered by unit tests
• Describe what the old behavior was prior to the change
• Describe the steps required to verify your changes are working as expected. Exclusively stating 'Specs run' is NOT acceptable as appropriate testing
• If this work is behind a flipper:
  • Tests need to be written for both the flipper on and flipper off scenarios. Docs.
  • What is the testing plan for rolling out the feature?
    This feature will be tested extensively on staging before release. It will be rolled out incrementally based on a percentage of production users, starting at 1%. All 0781 upload attempts will be logged and monitored.

Acceptance criteria

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No error nor warning in the console.
• Events are being sent to the appropriate logging solution - This will be added later
• Documentation has been updated (link to documentation)
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Feature/bug has a monitor built into Datadog (if applicable) - This will be added later
• If app impacted requires authentication, did you login to a local build and verify all authenticated routes work as expected
• I added a screenshot of the developed feature

[ajones446]

Re-do in #19186