feat(maasservicelayer): add bulk-remove and bulk-create operations for users and entitlements in groups

[nehjoshi]

Added 3 new endpoints:

1. POST /groups/{group_id}/members:batch_create: For adding multiple users to a group in one operation. The batch_create suffix is appended to separate this endpoint from the POST /groups/{group_id}/members for adding a single member.
2. DELETE /groups/{group_id}/members: For removing multiple users from a group in one operation
3. POST /groups/{group_id}/entitlements:batch_delete: Removes multiple entitlements from a group in a single operation. Entitlements within a group do not have a unique id. Instead, each entitlement is identified by a combination of fields: resource_type, resource_id, and entitlement (e.g. can_edit_machines). Deleting multiple entitlements therefore requires sending a structured payload (e.g. an array of these field combinations).
   While this could map to a DELETE endpoint, doing so would require a request body. In practice, request bodies on DELETE are discouraged by MDN (see docs). For this reason, POST is chosen. Also, the suffix batch_delete is appended because POST /groups/{group_id}/entitlements already exists for entitlement creation.

The following endpoints are modified and now include pagination:

1. GET /groups/{group_id}/members
2. GET /groups/{group_id}/entitlements

Resolves MAASENG-6347

[r00ta]

In general I discourage to pass list[tuple[str,str,int]]. Use a typed model instead