Allow lower-case SD values and also allow 0xSD values in config

Changes

• ignoring upper/lowercase of SD value
• Supporting leading 0x of SD value
• do NOT support decimal values

Now the following SD configurations work

snssais:
  - &embb_slice1
    sst: 1
    sd: 0xFFFfff
  - &embb_slice2
    sst: 2
    sd: 00ba20

@luis_pereira87 please have a look if that works now

common-src MR oai-cn5g-common-src!70 (merged)

@defosseu Relates to oai-cn5g-fed!172