Add option to disable bulk operations for each server

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Feature request

Which Nextcloud Version are you currently using: master (33.0.0)

Is your feature request related to a problem? Please describe.
Now that bulk support is no longer required as of #109, it might be nice to have the option to not sync users/groups in bulk even if the server supports it.

Describe the solution you'd like
Add to each SCIM server a toggle that enables/disables the use of bulk operations for that server. That way, bulk operations are only used if it is enabled in both the app and the server.

Describe alternatives you've considered
This may not be completely necessary as a workaround would be to toggle bulk support in the SCIM server itself, but could be helpful in situations when bulk support cannot be disabled from the server side.

Additional context
(none)

No files or tests are named. Start by locating the SCIM server configuration and the code paths that choose bulk user or group operations; then trace how server settings are stored and exposed. Done means each server can independently enable or disable bulk operations, with bulk used only when both relevant settings allow it.