[Yang Models] Add Yang Model and tests for VNET_ROUTE_TUNNEL table

[miatttao]

Why I did it

Added YANG model and tests for VNET_ROUTE_TUNNEL table to provide schema validation and testing capabilities for VNET route tunnel configurations.

Work item tracking

• Microsoft ADO (number only):

How I did it

• Created new YANG model (sonic-vnet-route-tunnel-vxlan.yang) defining the VNET_ROUTE_TUNNEL table schema
• Added comprehensive test cases covering both valid and invalid configurations
• Implemented validation for:
  • Mandatory endpoint field
  • MAC address format
  • VXLAN ID range
  • Name format (vnet_name:prefix)
  • Reference integrity with VXLAN configurations

How to verify it

1. Review the YANG model against the config_db schema
2. cd src/sonic-yang-models && python3 setup.py test

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

https://github.com/Azure/sonic-buildimage/blob/master/src/sonic-yang-models/doc/Configuration.md/VNET_ROUTE_TUNNEL

A picture of a cute animal (not mandatory but encouraged)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: miatttao (5e394d9, a68a9ab, 8367814)

[mssonicbld]

/azp run Azure.sonic-buildimage

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

/azp run Azure.sonic-buildimage

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

/azp run Azure.sonic-buildimage

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).