[Enhancement]: Allow users to select a provider when migrating site to a shared server

### Affected Area

None

### Problem Statement

Users currently cannot choose a provider of their choice when they want to migrate to a shared server.

### Proposed Solution

If users choose 'Shared server' under 'Select server type', show an additional dropdown selection with all available providers for shared servers.

The issue names no files or tests. Start by locating the migration flow's “Select server type” control and the data source for shared-server providers. Done means a provider dropdown appears for “Shared server” and the selected provider is retained and used during migration.

Written by the indexing model from the issue text.