templates: add netmiko_device_type for automation tools

Common template field (schema + templates table column + Alembic
migration) holding the Netmiko device type (e.g. 'cisco_xr', 'nokia_srl')
so Netmiko/Nornir based tooling can look up how to reach a node's CLI
without hard-coded vendor mappings. Free-form lowercase string on
purpose: Netmiko's platform list evolves independently of GNS3.

gns3server/schemas/controller/templates/__init__.py

@@ -48,6 +48,11 @@ class TemplateBase(BaseModel):
     template_type: Optional[NodeType] = None
     compute_id: Optional[str] = None
     usage: Optional[str] = ""
+    netmiko_device_type: Optional[str] = Field(
+        None,
+        description="Device type for Netmiko-based automation tools (e.g. 'cisco_xr' or 'nokia_srl')",
+        pattern=r"^[a-z0-9_]+$",
+    )
     tags: Optional[List[str]] = Field(
         default_factory=list,
         description="User-defined metadata tags (e.g. 'vendor:cisco' or 'model:7200')"