Add the add_delegate validate code reason

[allgandalf]

Adds the add_delegate reason so App can pass it when you invite a copilot. The backend half is https://github.com/Expensify/Auth/pull/24274.

Fixed Issues

$ https://github.com/Expensify/Expensify/issues/679000

Tests

1. None, this is a constant.
2. Added it to the installed copy in App and confirmed [HOLD Auth 24274][HOLD expensify-common 940] Send the add_delegate validate code reason App#100442 typechecks against it.

QA

1. Nothing, no behaviour change until App uses it.
2. None.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[allgandalf]

I have read the CLA Document and I hereby sign the CLA

[melvin-bot]

@ looks like this was merged without a test passing. These checks had not passed on b88f289 when it was merged:

• Check independent approval (failure)

Please add a note explaining why this was done and remove the Emergency label if this is not an emergency.

[carlosmiceli]

Mmmm, no, tests showed all passing when I merged.