[dnssd] SRP - supports subtype

[Kangping Dong <[e-mail]>]

Hi,

Seems project CHIP
<https://github.com/CHIP-Specifications/connectedhomeip-spec/blob/master/src/secure_channel/Discovery.adoc>
needs support of "subtype":
>
> Three subtypes for CHIP Commissioning Discovery are defined:
>
>    1.
>
>    Sddd (eight-bit short discriminator, encoded as a three-digit decimal
>    number in ASCII (UTF-8) text, with leading zeroes as necessary)
>    2.
>
>    Ldddd (twelve-bit long discriminator, encoded as a four-digit decimal
>    number in ASCII (UTF-8) text, with leading zeroes as necessary)
>    3.
>
>    Vddd (optional vendor id, encoded as a variable-length decimal number
>    in ASCII (UTF-8) text, without leading zeroes)
>
> (Please search "subtype" in this doc
<https://github.com/CHIP-Specifications/connectedhomeip-spec/blob/master/src/secure_channel/Discovery.adoc>
to
find the source

But current SRP spec seems not supporting multiple PTRs (Service Discovery
Instructions):

> An SRP Update MUST include at zero or more Service Discovery
>    Instructions, the same number of Service Description Instructions,
>    and exactly one Host Description Instruction.
>
> (https://tools.ietf.org/html/draft-ietf-dnssd-srp-08#section-2.3.2

I think multiple Service Discovery Instructions need to be allowed for a
single Service Description Instruction, if we want to support subtype.
Thoughts?

BRs,
Kangping