Add canLoginAs & canMergeUsers to /users endpoint

[jardakotesovec]

Describe the issue

User table is being migrated to the Vue.js, relying on the /users endpoint to list the users.

Its not possible to calculate more complex permissions on client side, therefore adding that to the API similarly to #10290 is probably reasonable solution.

LoginAs

LoginAs reference implementation: https://github.com/pkp/pkp-lib/blob/main/controllers/grid/settings/user/UserGridRow.php#L176

Merge users

Merge user reference implementation: https://github.com/pkp/pkp-lib/blob/main/controllers/grid/settings/user/UserGridRow.php#L198

Performance considerations

If its not feasible to optimise the current calculation, that requires multiple sql queries per user - we would want to hide it behind include query param to not affect other use cases for /users endpoint. Otherwise it could be included by default

What application are you using?
OJS, OMP or OPS version 3.5