Tracking refactors related to PR #4405

[rysweet]

This issue is collecting refactors/bugs/tests noted in Kostas big PR #4405 which became too big/too old to rebase and didn't track enough changes in main.
Making a list here to prioritize bringing changes over.

• Move AgentBase -> Agent #4667 - see refactor renaming agent base - 4667 #4669
• remove IAgentBase #4671 - see Rysweet 4671 remove iagent base #4673
• Abstractions -> Contracts #4670 - see Rysweet refactor 4670 rename abstractions to contracts #4674
• bugs in RequestAsync #4672 - see Rysweet refactor 4670 rename abstractions to contracts #4674
• Remove Orleans from abstractions/contracts #4675 - see Rysweet refactor 4670 rename abstractions to contracts #4674
• move GRPC services to Runtime.Grpc which depends only on Contracts #4679 - see Rysweet 4679 move grpc to runtimegrpc project #4680
• ReflectionHelper, EventTypes broken out into its own class #4687 - see Rysweet 4679 move grpc to runtimegrpc project #4680
• Core doesn't use/depend on Orleans #4688
• rename Agents package -> Core which depends only on contracts #4677
• Remove IAgentRuntime/collapse into IAgentWorker #4676
• move service loop out of constructor or GrpcGatewayService #4700
• IHandle, IHandleExtensions in Core #4701 - see rysweet-4677-rename-agents-project-to-core-trim-dependencies #4696
• add microsoft.autogen.agenthost package #4702 - see Rysweet 4702 add microsoftautogenagenthost package #4703
• bring over the tests from pr Refactor and cleanup of dotnet codebase #4405
• doc comments everywhere