feat: add AgentAPI using DRPC (#10811)

Co-authored-by: Spike Curtis <spike@coder.com>

coderd/agentapi/servicebanner.go

@@ -0,0 +1,38 @@
+package agentapi
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+
+	"golang.org/x/xerrors"
+
+	agentproto "github.com/coder/coder/v2/agent/proto"
+	"github.com/coder/coder/v2/coderd/database"
+	"github.com/coder/coder/v2/codersdk"
+)
+
+type ServiceBannerAPI struct {
+	Database database.Store
+}
+
+func (a *ServiceBannerAPI) GetServiceBanner(ctx context.Context, _ *agentproto.GetServiceBannerRequest) (*agentproto.ServiceBanner, error) {
+	serviceBannerJSON, err := a.Database.GetServiceBanner(ctx)
+	if err != nil && !xerrors.Is(err, sql.ErrNoRows) {
+		return nil, xerrors.Errorf("get service banner: %w", err)
+	}
+
+	var cfg codersdk.ServiceBannerConfig
+	if serviceBannerJSON != "" {
+		err = json.Unmarshal([]byte(serviceBannerJSON), &cfg)
+		if err != nil {
+			return nil, xerrors.Errorf("unmarshal json: %w, raw: %s", err, serviceBannerJSON)
+		}
+	}
+
+	return &agentproto.ServiceBanner{
+		Enabled:         cfg.Enabled,
+		Message:         cfg.Message,
+		BackgroundColor: cfg.BackgroundColor,
+	}, nil
+}