Implement support for a "tdns plugin" to MUSIC (the MUlti-SIgner Controller)

[johanix]

MUSIC has a rather simple interface requirement (as described in https://github.com/DNSSEC-Provisioning/music/blob/main/music/updater.go). It essentially boils down to implementing three API calls:

Update(), FetchRRset() and RemoveRRset().

Each of these deal with DNS RRs in [][]dns.RR. An "updater plugin" in MUSIC will need to implement the existing interface and translate that to API calls into TDNS-{SERVER,AGENT}. TDNS doesn't really use []dns.RR much and instead uses its own RRset struct. Whether the conversion between []dns.RR and RRset is done in the plugin or in the TDNS end doesn't really matter.

However, if the RRset struct is a better alternative than []dns.RR then that's an argument for perhaps even converting MUSIC to use RRset instead of []dns.RR, but that's a separate discussion.

In the TDNS end we need three new endpoints:

• /multisigner/update: MUSIC wants to send lists of dns.RRs to add and/or remove. TDNS has support for add/remove operations (as part of the DNS UPDATE support), so this is mostly a question of conversion of datastructures.
• /multisigner/fetch-rrset: return a single RRset from a zone that TDNS serves.
• /multisigner/remove-rrset: delete a single RRset from a zone that TDNS serves.

Then on the MUSIC side the actual plugin that accesses these endpoints need to be implemented. See DNSSEC-Provisioning/music#218