Add GetXxxClient extension methods for AI Foundry supported packages

[ShivangiReja]

• Move extension methods (such as AzureAIProjectsExtensions) into their respective packages (e.g., Search extensions should be moved to the Search package).
• Ensure that each package defines its extension methods in a type following the naming convention {PackageName}Extensions (e.g., SearchExtensions).
• Keep the extension method types in the top-level namespace within their respective packages.
• Update documentation and add tests to validate the new locations of these extensions.