Transform physical address to octet string

Signed-off-by: leo-ma <begeekmyfriend@gmail.com>

New API for physical address to octet string transformation.
E.X. In `interface.lua`
```
local utils = require "smartsnmp.utils"

entry = {
    phyaddr = utils.utils.mac2str('001f1633e721'),
    ...
}
```
This movement will transform mac address into 6-byte octet string (such as "Hex String = 00 1f 16 33 e7 21").