Design doc for portable console switch for multiple vendors

[lizhijianrd]

In this design document, we provide the standard for adapting console devices to SONiC. If a vendor's console device implementation conforms to this standard, it will work on any switch with SONiC installed.

[zhangyanzhao]

@lizhijianrd several things need be updated:

1. Please define YANG table for the new config_DB table
2. Please add the new CLIs to https://github.com/sonic-net/sonic-utilities/blob/master/doc/Command-Reference.md
3. Please add the code PRs into this HLD by referring to PR#806
4. Please make sure to have the UT defined

[lizhijianrd]

@lizhijianrd several things need be updated:

1. Please define YANG table for the new config_DB table
2. Please add the new CLIs to https://github.com/sonic-net/sonic-utilities/blob/master/doc/Command-Reference.md
3. Please add the code PRs into this HLD by referring to PR#806
4. Please make sure to have the UT defined

Thanks @zhangyanzhao! Currently we are still in the design phase, so I'll update these points when I begin to implementation.