Add DI extensions for Azure and SCM-based clients

[annelo-msft]

Now that Microsoft.Extensions namespaces are considered part of the BCL, we can simplify our story around how Azure and SCM-based clients can be added to DI service collections.

I have a sketch showing how this may work in this prototype PR: #46593

Remaining work here includes:

• Close on IConfiguration schema/story
• Validate this approach will work for Azure clients - need to list functional scenarios supported by Microsoft.Extensions.Azure and ensure they can be implemented via new extensions
• Finalize API shapes and patterns

Once we make a go/no-go decision here, we'll want to solve back-compat with Microsoft.Extensions.Azure packages and make a plan for updating docs.