Move DurableTaskRegistry to Abstractions

[jviau]

Overview

This PR moves DurableTaskRegistry to Abstractions assembly. The reason for this move is for the code-gen scenario. It is important to have access to the registry from just the abstractions package, so registration methods can be generated.

Summary of Changes

• DurableTaskRegistry moved to Abstractions assembly.
• DurableTaskRegistry.Build moved to an extension method BuildFactory in worker assembly.
• Internal dictionaries on the registry were changed to IDictionary.
• Systems.Collection.Immutable removed from Abstractions.
  • Systems.Collection.Immutable was added to Worker.Grpc, but that is already used by another nuget package. This addition is to resolve a version mismatch.
• System.Collections.Concurrent removed - it was unused.
• Abstractions internals were made visible to Worker and Client projects.
  • Not the gRPC implementations, so we do not accidentally depend on functionality other worker implementations would not have access to.
  • Had to adjust some shared sections to avoid type duplication.

[cgillum]

Overall, LGTM!