Allow specifying device ID to User Login Admin API

[anoadragon453]

Being able to specify the device ID when using the User Login Admin API when logging in to a user's account is beneficial for several reasons:

• It allows the user to identify admin tooling based on its device ID.
• It allows admin tooling to log in to the same device ID repeatedly (i.e. to ensure E2EE continuity between sessions).

Currently the User Login Admin API specifically does not create a device:

This API does not generate a new device for the user, and so will not appear their /devices list, and in general the target user should not be able to tell they have been logged in as.

This is useful for Trust & Safety teams in order to allow them to log in as a user and execute requests without alerting the user to what's going on.

Proposal

A device_id body field is added to the User Login Admin API, for which the value is a string specifying the new device's ID.

By default, if the field is not present, a device is not created (as is the case today). If the field is present, then a device is created.

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Note: the behaviour of not creating a device is marked as deprecated in the codebase.