Update TypeProvider

[live1206]

Needed for Azure/azure-sdk-for-net#47665

• Update modifiers of TypeProvider
• Update relativeFilePath of TypeProvdier
• Make StringExtensions public so that sub-plugins don't need to re-implement
  • move StringExtensions to Microsoft.Generator.CSharp.Input and remove the shared link of it in others
• remove sealed from ModelProvider, so that ResourceDataProvdier in Azure plugin can share most of the existing logic

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

Microsoft.Generator.CSharp
Microsoft.Generator.CSharp.Input

[ArcturusZhang]

shall we consider to make the stringextensions as shared file that is imported by azure plugin?
maybe not quite a good idea but we could consider if we prefer not making it public

[live1206]

shall we consider to make the stringextensions as shared file that is imported by azure plugin? maybe not quite a good idea but we could consider if we prefer not making it public

Since importing StringExtensions to Azure plugin is across-repo, we need to maintain additional scripts or manual effort to update it to latest.

Not sure about the reasons of not making it public, I will hold the merge until we get this clarified.
cc @JoshLove-msft