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/db/models/templates.py

@@ -35,6 +35,7 @@ class Template(BaseTable):
     symbol = Column(String)
     builtin = Column(Boolean, default=False)
     usage = Column(String)
+    netmiko_device_type = Column(String)
     template_type = Column(String)
     tags = Column(JSON)
     compute_id = Column(String)