[202205][chery-pick] Add Yang model for SYSTEM_DEFAULTS table

[bingwang-ms]

Why I did it

This PR is to backport PR #11117 into 202205 branch.
This PR is to define Yang model for SYSTEM_DEFAULTS table.
The table was introduced in PR sonic-net/SONiC#982
The table will be like

"SYSTEM_DEFAULTS": {
        "tunnel_qos_remap": {
            "status": "enabled"
        }
}

Work item tracking

• Microsoft ADO (https://msazure.visualstudio.com/One/_workitems/edit/23037078)

How I did it

Add a new yang file sonic-system-defaults. Yang.

How to verify it

Verified by UT

collected 3 items                                                                                                                                                                                     

tests/test_sonic_yang_models.py ..                                                                                                                                                              [ 66%]
tests/yang_model_tests/test_yang_model.py .                                                                                                                                                     [100%]

====================================================================================== 3 passed in 2.27 seconds =======================================================================================

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

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

Tested branch (Please provide the tested image version)

Description for the changelog

Add Yang model for SYSTEM_DEFAULTS table.

Link to config_db schema for YANG module changes

https://github.com/sonic-net/sonic-buildimage/blob/master/src/sonic-yang-models/doc/Configuration.md#systemdefaults-table

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