feat: Add --server option to microk8s config for custom hostname (#5270)

[Ray0907]

Allow users to specify a custom hostname/IP for microk8s config output
using --server flag or persistent config file at
$SNAP_DATA/args/config-server.

Supports IPv4, IPv6 (bracketed), and optional custom port.

Summary

Add --server option to microk8s config command, allowing users to
specify
a custom hostname or IP address for the cluster server output instead of
manually replacing IPs after each command.

Closes #5270

Changes

• Add -s/--server HOST option to specify custom hostname or IP address
• Support host:port format (e.g., --server k8s.example.com:6443)
• Support IPv6 addresses with brackets (e.g., --server [::1]:6443)
• Add persistent configuration via $SNAP_DATA/args/config-server file
• Add input validation with helpful error messages
• Add conflict detection when --use-loopback and --server are used
  together
• Fix edge case when get_default_ip returns "none"

User-facing changes:

• New --server flag for microk8s config
• New config file $SNAP_DATA/args/config-server for persistent
  settings

Testing

Manual testing performed:

• Verified microk8s config --server hostname outputs correct config
• Verified microk8s config --server hostname:port replaces both host
  and port
• Verified microk8s config --server [::1]:6443 works correctly for
  IPv6
• Verified persistent config file is read when present
• Verified --server flag takes precedence over config file
• Verified --use-loopback still works as before
• Verified error handling for invalid inputs

Possible Regressions

None expected. Existing behavior is preserved:

• Default behavior (using default IP) unchanged
• --use-loopback flag works as before

Checklist

• Read the [contributions](https://github.com/canonical/microk8s/blo
  b/master/CONTRIBUTING.md) page.
• Submitted the CLA
  form, if you are a
  first time contributor.
• The introduced changes are covered by unit and/or integration
  tests.

Notes

Priority order for server address:

1. --server CLI flag (highest)
2. $SNAP_DATA/args/config-server file
3. --use-loopback flag
4. Default interface IP (lowest)

The cross-node replication mentioned in #5270 is not implemented in this
PR,
as microk8s config is typically only run on the master node (worker
nodes
are blocked by clustered.lock). This could be a future enhancement if
needed.