pldmtool: Print all available commands in GetPLDMCommands output

Currently GetPLDMCommands command prints only the commands for which
it has a description mapping. Support printing types without the
description mapping as well.

Tested:
Currently pldmtool doesn't have a description for the PLDM commands
from the "PLDM for Redfish Device Enablement" specification (PLDM type
6), therefore a device with a support for such type was used for
testing purposes.

Before:
pldmtool: Tx: 80 00 05 06 00 f0 f1 f1
pldmtool: Rx: 00 00 05 00 fe 0f 7b 00 00 00 03 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[
    null,
    null,
    <...>
    null,
    null
]

After:
pldmtool: Tx: 80 00 05 06 00 f0 f1 f1
pldmtool: Rx: 00 00 05 00 fe 0f 7b 00 00 00 03 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[
    {
        "PLDM Command": "unknown",
        "PLDM Command Code": 1
    },
    {
        "PLDM Command": "unknown",
        "PLDM Command Code": 2
    },
    <...>
    {
        "PLDM Command": "unknown",
        "PLDM Command Code": 48
    },
    {
        "PLDM Command": "unknown",
        "PLDM Command Code": 49
    }
]

Change-Id: Ia309ae1b21c38b95c889023b44b8c69b66fa39dd
Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>