[config] parse optional fields from hwsku

[dmytroxshevchuk]

- Why I did it
For now hwsku.json and platform.json dont support optional fields. For example no way to add fec or autoneg field using platform.json and hwsku.json.

- How I did it
Added parsing of optional fields from hwsku.json.

- How to verify it
Add optional field to hwsku.json. After first boot will be generated new config_db.json or you can generate it using sonic-cfggen command. In this file must be optional field from hwsku.json or check using command redis-cli hgetall PORT_TABLE:Ethernet0
Example of hwsku.json, that must be parsed:

{
    "interfaces": {
        "Ethernet0": {
            "default_brkout_mode": "1x100G[40G]",
            "fec": "rs",
            "autoneg": "0"
        },
...
}

Example of generated config_db.json:

    "PORT": {
        "Ethernet0": {
            "alias": "Ethernet0",
            "lanes": "0,1,2,3",
            "speed": "100000",
            "index": "1",
            "admin_status": "up",
            "fec": "rs",
            "autoneg": "0",
            "mtu": "9100"
        },

So, we can see this entries in redis db:

admin@sonic:~$ redis-cli hgetall PORT_TABLE:Ethernet0

 1) "alias"
 2) "Ethernet0"
 3) "lanes"
 4) "0,1,2,3"
 5) "speed"
 6) "100000"
 7) "index"
 8) "1"
 9) "admin_status"
10) "up"
11) "fec"
12) "rs"
13) "autoneg"
14) "0"
15) "mtu"
16) "9100"
17) "description"
18) ""
19) "oper_status"
20) "up"

Also its way to fix show interface status, FEC field but also need add FEC field to hwsku.json.
Before:

admin@sonic:~$ show interfaces status
  Interface            Lanes    Speed    MTU    FEC        Alias    Vlan    Oper    Admin             Type    Asym PFC
-----------  ---------------  -------  -----  -----  -----------  ------  ------  -------  ---------------  ----------
  Ethernet0          0,1,2,3     100G   9100     N/A    Ethernet0  routed      up       up  QSFP28 or later         N/A

After:

admin@sonic:~$ show interfaces status
  Interface            Lanes    Speed    MTU    FEC        Alias    Vlan    Oper    Admin             Type    Asym PFC
-----------  ---------------  -------  -----  -----  -----------  ------  ------  -------  ---------------  ----------
  Ethernet0          0,1,2,3     100G   9100     rs    Ethernet0  routed      up       up  QSFP28 or later         N/A

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

• 201811
• 201911
• 202006

- Description for the changelog

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