Add interactive capacity_migration UI

[Copilot]

Adds labs.capacity_migration(), an anywidget-based UI (styled after perspective_editor) that drives the workflows in the Capacity Migration.ipynb notebook for migrating P/A SKU capacities to F SKU capacities.

UI

• Three tabs matching the supported scenarios:
  • Single capacity — pick a P/A source, then either create a new F SKU (live preview of target name, mapped F SKU size, region, admins, and whether the target already exists) or reassign workspaces to an existing F SKU.
  • Multiple capacities — multi-select P/A SKUs with a P SKUs only toggle.
  • All P/A capacities — region-grouped summary + P SKUs only toggle.
• Cross-region guard: the existing-F-SKU dropdown is filtered to the source's region; a Python-side check rejects mismatched regions before invoking assign_workspaces_to_capacity.
• Confirmation prompt before each migration, refresh button (re-queries list_capacities), inline status panel, dark/light toggle.

Wiring

• Source list_capacities() powers the source/target dropdowns; icons.sku_mapping and icons.migrate_capacity_suffix drive the target preview.
• Single-create / multiple / all paths call migrate_capacities(...); single-existing calls assign_workspaces_to_capacity(...).
• Optional Service Principal Authentication section — when enabled, each action is wrapped in service_principal_authentication(...). Initial values can be pre-populated from function arguments so the existing notebook variables carry through.

Files

• src/sempy_labs/_capacity_migration_ui.py — new module.
• src/sempy_labs/__init__.py — export capacity_migration.
• notebooks/Capacity Migration.ipynb — new section demonstrating the UI:

labs.capacity_migration(
    azure_subscription_id=azure_subscription_id,
    resource_group=resource_group,
    key_vault_uri=key_vault_uri,
    key_vault_tenant_id=key_vault_tenant_id,
    key_vault_client_id=key_vault_client_id,
    key_vault_client_secret=key_vault_client_secret,
)