Mock SignETS API to enable testing with test-only accounts

**Is your feature request related to a problem? Please describe.**
As a developer, I would love to be able to test bugs and edge cases with different test accounts instead of using mine.

**Describe the solution you'd like**
It would be great to mock the SignETS API on debug mode to use test accounts without the need to ask the IT service to create/manage accounts for us. This would help us debug the app when our (real) user accounts don't let us reproduce or test issues.

**Describe alternatives you've considered**
The easiest solution we considered so far is to host a server (locally or online) with a SOAP API like SignETS' that would respond to API calls with mocked data to emulate different behaviors or use cases.
We could integrate a toggle in the app (debug mode only) to choose whether to use SignETS or the mocked API.
The endpoint to use for mocked API calls could be set in the `.env` file.

**Additional context**
@MysticFragilist has a Postman project with the structure of SignETS' API that could help us reproduce a similar API.