feat: receive decline_reason from payload, save to database, and incl…

[msaha-sonata]

Description:
Backend: B&R Decline | Decline reason field
Receive decline_reason from payload, save to database, and include in API response

Jira:
ENT-11190

Merge checklist:

• ./manage.py makemigrations has been run
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance

[iloveagent57]

Seems reasonable, but you should update the unit test for the view to include an assertion that the decline reason is stored in the model record.

[msaha-sonata]

Opened a new PR to clean branches linked
New PR - #25