Remove input type checking in favour of explicit `.DescribeProtocolAsync()` flow. Also removes `.AsAgentAsync()` as the validation happens at workflow run time. This makes it easier to use Workflows with DI without resorting to async-over-sync.
* refactor: remove unused internals
* feat: Execution Mode for sharing a workflow among concurrent runs
* feat: Update WorkflowHostAgent to support concurrent execution
* Also update AsAgent APIs to support injecting a CheckpointManager and an IWorkflowExecutionEnvironment
* fix: Make Read logic consistent in DeclarativeWorkflowContext
WorkflowHostAgent was initially implemented before Checkpointing was available. This meant that in order to support resuming, the WorkflowHostAgent needed to keep the runs around, which broke it when stricter rules about concurrent sharing of workflows during execution were introduced.
This change updates the hosting logic to release the underlying StreamingRun when the RunStreamingAsync or RunAsync are invoked, in favour of keeping the checkpointing information in the WorkflowThread to enable resumption.