feat: add Durable Task Scheduler (DTS) backend

Add a new backend/scheduler package that implements the Backend interface
for connecting to the Durable Task Scheduler cloud service over gRPC.

Key components:
- SchedulerOptions: configuration with endpoint, task hub, auth type
- Connection string parsing matching the .NET SDK format
  (Endpoint=...;TaskHub=...;Authentication=...)
- gRPC channel with TLS auto-detection and metadata interceptors
  (taskhub, workerid headers injected on every request)
- Stub Backend interface implementation (only used for connection)
- Worker identity generation (hostname,pid,uuid) matching existing backends

The DTS backend is a remote service, so work item dispatch is handled
through the gRPC streaming protocol rather than local polling.

Includes unit tests for connection string parsing and backend construction.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: halspang

backend/durabletaskscheduler/options.go

@@ -0,0 +1,101 @@
+package durabletaskscheduler
+
+import (
+	"fmt"
+	"strings"
+)
+
+const (
+	// DefaultResourceID is the default Azure resource ID used for token authentication with DTS.
+	DefaultResourceID = "https://durabletask.io"
+
+	// DefaultAuthType is the default authentication type.
+	DefaultAuthType = AuthTypeDefaultAzure
+)
+
+// AuthType represents the authentication type for connecting to DTS.
+type AuthType string
+
+const (
+	// AuthTypeNone disables authentication (for local development).
+	AuthTypeNone AuthType = "none"
+
+	// AuthTypeDefaultAzure uses DefaultAzureCredential from the Azure Identity SDK.
+	AuthTypeDefaultAzure AuthType = "defaultazure"
+)
+
+// Options configures the connection to a Durable Task Scheduler (DTS) endpoint.
+type Options struct {
+	// EndpointAddress is the DTS endpoint URL (e.g., "https://myscheduler.westus2.durabletask.io").
+	EndpointAddress string
+
+	// TaskHubName is the name of the task hub resource.
+	TaskHubName string
+
+	// AuthType specifies how to authenticate with the DTS service.
+	AuthType AuthType
+
+	// ResourceID is the Azure resource ID used for token scoping. Defaults to "https://durabletask.io".
+	ResourceID string
+}
+
+// NewOptions creates a new Options with the given endpoint and task hub name.
+func NewOptions(endpointAddress, taskHubName string) *Options {
+	return &Options{
+		EndpointAddress: endpointAddress,
+		TaskHubName:     taskHubName,
+		AuthType:        DefaultAuthType,
+		ResourceID:      DefaultResourceID,
+	}
+}
+
+// NewOptionsFromConnectionString parses a connection string into Options.
+//
+// The connection string format is:
+//
+//	Endpoint=https://{scheduler-name}.{region}.durabletask.io;TaskHub={taskhub-name};Authentication={auth-type}
+//
+// Required keys: Endpoint, TaskHub.
+// Optional keys: Authentication (defaults to "defaultazure").
+func NewOptionsFromConnectionString(connectionString string) (*Options, error) {
+	opts := &Options{
+		AuthType:   DefaultAuthType,
+		ResourceID: DefaultResourceID,
+	}
+
+	parts := strings.Split(connectionString, ";")
+	for _, part := range parts {
+		part = strings.TrimSpace(part)
+		if part == "" {
+			continue
+		}
+
+		kv := strings.SplitN(part, "=", 2)
+		if len(kv) != 2 {
+			return nil, fmt.Errorf("invalid connection string segment: %q", part)
+		}
+
+		key := strings.TrimSpace(kv[0])
+		value := strings.TrimSpace(kv[1])
+
+		switch strings.ToLower(key) {
+		case "endpoint":
+			opts.EndpointAddress = value
+		case "taskhub":
+			opts.TaskHubName = value
+		case "authentication":
+			opts.AuthType = AuthType(strings.ToLower(value))
+		default:
+			// Ignore unknown keys for forward compatibility
+		}
+	}
+
+	if opts.EndpointAddress == "" {
+		return nil, fmt.Errorf("connection string is missing required 'Endpoint' key")
+	}
+	if opts.TaskHubName == "" {
+		return nil, fmt.Errorf("connection string is missing required 'TaskHub' key")
+	}
+
+	return opts, nil
+}