[sled-agent] Replace tokio::spawn'd mg-ddm calls with a reconciler task

[jgallagher]

Prior to this PR, sled-agent would spawn three infinite retry-until-success tasks on startup:

1. Sending mg-ddm enough information to turn on its Oximeter stats
2. Telling mg-ddm to advertise the sled's bootstrap prefix
3. Telling mg-ddm to advertise the sled's underlay prefix

Additionally, it would spawn an infinite retry-until-success task for any internal DNS zone it started (telling mg-ddm to advertise the internal DNS prefix).

This PR replaces all of these with a DdmReconciler, which is responsible for taking all of the above information and periodically syncing with the local mg-ddm instance to ensure it has up-to-date information.

Fixes #7377. Builds on #7507.

[jgallagher]

Notes from testing on a4x2: On sled startup, we see each of the bootstrap, underlay, and internal-dns prefixes added in turn:

19:11:37.635Z INFO SledAgent (DdmReconciler): attempting to add new prefix advertisements
    file = sled-agent/src/ddm_reconciler.rs:219
    to_advertise = [Ipv6Net { addr: fdb0:a840:2500:1::, width: 64 }]
19:11:37.659Z INFO SledAgent (DdmReconciler): completed prefix advertisement reconcilation
    added = [Ipv6Net { addr: fdb0:a840:2500:1::, width: 64 }]
    file = sled-agent/src/ddm_reconciler.rs:251
    prefixes = {Ipv6Net { addr: fdb0:a840:2500:1::, width: 64 }}
    withdrawn = []
... snip ...
19:14:31.940Z INFO SledAgent (DdmReconciler): attempting to add new prefix advertisements
    file = sled-agent/src/ddm_reconciler.rs:219
    to_advertise = [Ipv6Net { addr: fd00:1122:3344:101::, width: 64 }]
19:14:31.943Z INFO SledAgent (DdmReconciler): completed prefix advertisement reconcilation
    added = [Ipv6Net { addr: fd00:1122:3344:101::, width: 64 }]
    file = sled-agent/src/ddm_reconciler.rs:251
    prefixes = {Ipv6Net { addr: fd00:1122:3344:101::, width: 64 }, Ipv6Net { addr: fdb0:a840:2500:1::, width: 64 }}
    withdrawn = []
... snip ...
19:15:04.817Z INFO SledAgent (DdmReconciler): attempting to add new prefix advertisements
    file = sled-agent/src/ddm_reconciler.rs:219
    to_advertise = [Ipv6Net { addr: fd00:1122:3344:1::, width: 64 }]
19:15:04.817Z INFO SledAgent (DdmReconciler): completed prefix advertisement reconcilation
    added = [Ipv6Net { addr: fd00:1122:3344:1::, width: 64 }]
    file = sled-agent/src/ddm_reconciler.rs:251
    prefixes = {Ipv6Net { addr: fd00:1122:3344:1::, width: 64 }, Ipv6Net { addr: fd00:1122:3344:101::, width: 64 }, Ipv6Net { addr: fdb0:a840:2500:1::, width: 64 }}
    withdrawn = []

Expunging the internal-dns zone results in its prefix being withdrawn:

21:51:15.823Z INFO SledAgent (DdmReconciler): attempting to withdraw prefix advertisements
    file = sled-agent/src/ddm_reconciler.rs:236
    to_withdraw = [Ipv6Net { addr: fd00:1122:3344:1::, width: 64 }]
21:51:15.856Z INFO SledAgent (DdmReconciler): completed prefix advertisement reconcilation
    added = []
    file = sled-agent/src/ddm_reconciler.rs:251
    prefixes = {Ipv6Net { addr: fd00:1122:3344:101::, width: 64 }, Ipv6Net { addr: fdb0:a840:2500:1::, width: 64 }}
    withdrawn = [Ipv6Net { addr: fd00:1122:3344:1::, width: 64 }]

and adding it back results in it being advertised again:

21:54:16.328Z INFO SledAgent (DdmReconciler): attempting to add new prefix advertisements
    file = sled-agent/src/ddm_reconciler.rs:219
    to_advertise = [Ipv6Net { addr: fd00:1122:3344:1::, width: 64 }]
21:54:16.349Z INFO SledAgent (DdmReconciler): completed prefix advertisement reconcilation
    added = [Ipv6Net { addr: fd00:1122:3344:1::, width: 64 }]
    file = sled-agent/src/ddm_reconciler.rs:251
    prefixes = {Ipv6Net { addr: fd00:1122:3344:1::, width: 64 }, Ipv6Net { addr: fd00:1122:3344:101::, width: 64 }, Ipv6Net { addr: fdb0:a840:2500:1::, width: 64 }}
    withdrawn = []