[mellanox] add D100C12S2 sku to SN4600C

[alexrallen]

Why I did it

New SKU by customer request.

4   100G Downlinks
2   10G  Downlinks
100 50G  Downlinks (2x Split)
8   100G Uplinks

VXLAN Source Port: Yes
Shared Headroom: Yes
2km Cables: Yes

How I did it

Duplicated existing 4600C SKU and updated

• Port Speeds
• Buffer Default Values
• Default Breakout Modes
• SAI Profile Speeds & Breakout Capabilities

How to verify it

Tested on 4600C with hardware loopback connections. Ensure links go up and pass traffic.

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

Another PR will be submitted after merge against master for backport to 202106.

Description for the changelog

[mellanox] Add D100C12S2 SKU to 4600C

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