marker: fix PUT marker with an enabled-only body (bpf no longer required)

update_marker reused MarkerCreate, whose bpf is required, so a partial PUT like
{"enabled": false} was rejected with 422 "bpf field required". Add a MarkerUpdate
schema with every field optional (bpf included; capture_node_id and name are
create-only/path-driven and omitted) and use it for the PUT route — partial
updates now validate cleanly.

gns3server/schemas/__init__.py

@@ -20,7 +20,7 @@ from .common import ErrorMessage
 from .version import Version
 
 # Controller schemas
-from .controller.links import LinkCreate, LinkUpdate, Link, UDPPortInfo, EthernetPortInfo, LinkCapture, MarkerCreate, MarkerDefinitionCreate
+from .controller.links import LinkCreate, LinkUpdate, Link, UDPPortInfo, EthernetPortInfo, LinkCapture, MarkerCreate, MarkerUpdate, MarkerDefinitionCreate
 from .controller.computes import ComputeCreate, ComputeUpdate, ComputeVirtualBoxVM, ComputeVMwareVM, ComputeDockerImage, AutoIdlePC, Compute
 from .controller.templates import TemplateCreate, TemplateUpdate, TemplateUsage, Template
 from .controller.images import Image, ImageType