Add core agent business logic (#705)

## Summary

This PR adds core business logic for the new tink agent component. The agent will replace the tink worker and can therefore be considered a re-write. Re-writing the worker has allowed us to remove unsupported functionality such as multi-worker workflows that were implemented in the previous Tinkerbell versions as 'tasks'.

The core business logic leverages a transport to retrieve workflows. It communicates workflow progression via events. It runs workflows serially rejecting requests to run concurrently with an explicit `event.WorkflowRejected` event. It assumes enforcement of state machines will be performed by the server component it interacts with.

Finally, the logging around the agent has been significantly improved to reflect exactly what's happening as it transitions between actions.

## Relevant designs/PRs

* [v1alpha2 API](https://github.com/tinkerbell/roadmap/blob/main/design/20230222_tinkerbell_crd_refactor.md)
* [Failure reason and message handling](https://github.com/tinkerbell/roadmap/blob/main/design/20230313_action_failure_reason_communication.md)
* [Tink agent PoC](#680)

## Future work

Separate PRs will introduce container runtime and transport implementations. See the Tink agent PoC for a reference.

internal/agent/agent.go

@@ -0,0 +1,82 @@
+package agent
+
+import (
+	"context"
+	"errors"
+
+	"github.com/go-logr/logr"
+	"github.com/tinkerbell/tink/internal/agent/event"
+	"github.com/tinkerbell/tink/internal/agent/workflow"
+)
+
+// Agent is the core data structure for handling workflow execution on target nodes. It leverages
+// a Transport and a ContainerRuntime to retrieve workflows and execute actions.
+//
+// The agent runs a single workflow at a time. Concurrent requests to run workflows will have the
+// second workflow rejected with an event.WorkflowRejected event.
+type Agent struct {
+	Log logr.Logger
+
+	// ID is the unique identifier for the agent. It is used by the transport to identify workflows
+	// scheduled for this agent.
+	ID string
+
+	// Transport is the transport used by the agent for communicating workflows and events.
+	Transport Transport
+
+	// Runtime is the container runtime used to execute workflow actions.
+	Runtime ContainerRuntime
+
+	sem chan struct{}
+}
+
+// Start finalizes the Agent configuration and starts the configured Transport so it is ready
+// to receive workflows. On receiving a workflow, it will leverage the configured Runtime to
+// execute workflow actions.
+func (agent *Agent) Start(ctx context.Context) error {
+	if agent.ID == "" {
+		return errors.New("ID field must be set before calling Start()")
+	}
+
+	if agent.Transport == nil {
+		return errors.New("Transport field must be set before calling Start()")
+	}
+
+	if agent.Runtime == nil {
+		//nolint:stylecheck // Specifying field on data structure
+		return errors.New("Runtime field must be set before calling Start()")
+	}
+
+	agent.Log = agent.Log.WithValues("agent_id", agent.ID)
+
+	// Initialize the semaphore and add a resource to it ensuring we can run 1 workflow at a time.
+	agent.sem = make(chan struct{}, 1)
+	agent.sem <- struct{}{}
+
+	agent.Log.Info("Starting agent")
+	return agent.Transport.Start(ctx, agent.ID, agent)
+}
+
+// HandleWorkflow satisfies transport.
+func (agent *Agent) HandleWorkflow(ctx context.Context, wflw workflow.Workflow, events event.Recorder) error {
+	// sem isn't protected by a synchronization data structure so this is technically invoking
+	// undefined behavior - consider this a best effort to ensuring Start() has been called.
+	if agent.sem == nil {
+		return errors.New("agent must have Start() called before calling HandleWorkflow()")
+	}
+
+	select {
+	case <-agent.sem:
+		// Replenish the semaphore on exit so we can pick up another workflow.
+		defer func() { agent.sem <- struct{}{} }()
+		return agent.run(ctx, wflw, events)
+
+	default:
+		reject := event.WorkflowRejected{
+			ID:      wflw.ID,
+			Message: "workflow already in progress",
+		}
+		events.RecordEvent(ctx, reject)
+		return nil
+	}
+}