[sonic-utilities] Add user management CLI commands

Add comprehensive CLI interface for SONiC local user management:

- config user add/modify/delete - User lifecycle management
- config user add-ssh-key/remove-ssh-key - SSH key management
- config user import-existing - Import system users to SONiC
- show user [username] - Display user information and status
- show user roles - Display available roles and permissions

Features:
- Role-based access control (administrator/operator)
- Secure password handling with confirmation prompts
- SSH key format validation and management
- CONFIG_DB integration for persistent configuration
- Comprehensive input validation and error handling
- Complete test suite with unit and integration tests

Commands integrate with userd daemon for system-level user operations.

Author: manoharan-nexthop

config/main.py

@@ -58,6 +58,7 @@
 from . import kube
 from . import muxcable
 from . import nat
+from . import user
 from . import vlan
 from . import vxlan
 from . import plugins
@@ -1560,6 +1561,7 @@ def config(ctx):
 config.add_command(kube.kubernetes)
 config.add_command(muxcable.muxcable)
 config.add_command(nat.nat)
+config.add_command(user.user)
 config.add_command(vlan.vlan)
 config.add_command(vxlan.vxlan)