Move shared source from Generator.Shared to Azure.Core.Shared

[annelo-msft]

This change moves all shared source files except the CodeGen*Attribute files in Generator.Shared into Azure.Core.Shared. They are now mastered in azure-sdk-for-net, with this PR: Azure/azure-sdk-for-net#38286

[m-nash]

We need to change this somehow https://github.com/Azure/autorest.csharp/blob/feature/v3/src/AutoRest.CSharp/build/CodeGeneration.targets#L89.

We need to be aware of people using this outside of azure-sdk-for-net. I don't know how we can bundle these files such that they will not be broken but this definitely a test we will want to do / verify.

I believe as long as the package contains the Azure.Core.Shared folder like it does here https://www.npmjs.com/package/@autorest/csharp?activeTab=code we can probably drop the Generator.Shared folder all together although this might break people?

Perhaps we need to keep the files in the same folder they were before so everything can find everything exactly where it was before?

[m-nash]

Need to update DownloadShareSource to include all the new files that are moving to Azure.Core.

Since you are using an alpha version there are 2 spots here where you need to change git checkout to git checkout [your branch] so that the new files can be found.

[annelo-msft]

We need to change this somehow https://github.com/Azure/autorest.csharp/blob/feature/v3/src/AutoRest.CSharp/build/CodeGeneration.targets#L89.

I think we can leave this for now. This change doesn't move the CodeGen*Attribute files out of the directory.