[Core] Shared source analysis and transition

[jsquire]

Overview

Source shared between Azure.Core, client libraries, and the generator create a difficult cross-repository circular dependency that causes friction when doing generator work and when generating libraries. To combat this, we've agreed to move to a model where internal types are emitted directly by the generator into libraries as a private copy and types that may be desirable externally are rescoped public and remain in Azure.Core.

Scope of work

• Analyze shared source types and work with Krzysztof to determine which should be public Azure.Core types.
• Prepare a list of the shared source types that will remain internal and discuss code generation work with Michael.
• Coordinate on generator work and timing to ensure that the generator changes can accompany the Core changes.
• Mark the Azure.Core types as public and ensure needed test coverage, filling in an gaps.

Out of scope

• The generator implementation; that will be covered by a separate issue.

Success criteria

• All shared source types have been analyzed and we have a clear understanding of what will move to the generator versus what will be public.
• Generator work has been scoped, discussed, and completed so that internal types are emitted.
• Types moving to public in Azure.Core are made public and have adequate test coverage.
• The Azure.Core test suite continues to pass reliably.