Set up screenshot tests

[manuroe]

Here is the current plan:

• setup the environment, using https://github.com/cashapp/paparazzi
• explain in a doc how to test a new screen (and update existing test)
• test first screens. Maybe the FTUE screens could be a good fit, since this part is already very well covered by other test types.
• Ensure the CI runs the tests.

[ouchadam]

would be interesting to investigate using https://github.com/cashapp/paparazzi to avoid needing a real device/emulator (renders the UI via the layoutlib/design preview)

[ouchadam]

first iteration was to spike the tooling internal doc

we'll need follow up tickets to fully implement the decision and write some tests

[ouchadam]

completed with #7128 & #7129