checker: implement ShareKey to mutualise TLS probes across targets

A TLS probe result depends only on the set of endpoints actually dialed and
the probe knobs, never on which domain or service published them: the
observation is a map keyed by each endpoint's contract Ref
(host|port|effective SNI|STARTTLS|require). Implement sdk.ObservationSharer so
the host dials a host:port once and serves every target that resolves to the
same endpoint set, instead of re-handshaking per record. This is the
highest-value case among the checkers, since dane, xmpp, srv, dav, … all
funnel their endpoints into this single checker.

The share key sorts the endpoint Refs and folds in the probe timeout and the
cipher-enumeration flag, since both change what is collected (a tighter
timeout can fail a slow handshake; enumeration adds the Enum block). An empty
or unparseable entry set yields "" so the host falls back to per-target
caching.

go.sum

@@ -1,5 +1,5 @@
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+git.happydns.org/checker-sdk-go v1.11.0 h1:+hs8OpcgvRMAoWyfqxih/Q0KYYUI1R8oFxO4RKioMLk=
+git.happydns.org/checker-sdk-go v1.11.0/go.mod h1:aNAcfYFfbhvH9kJhE0Njp5GX0dQbxdRB0rJ0KvSC5nI=
 github.com/andybalholm/brotli v1.0.6 h1:Yf9fFpf49Zrxb9NlQaluyE92/+X7UVHlhMNJN2sxfOI=
 github.com/andybalholm/brotli v1.0.6/go.mod h1:fO7iG3H7G2nSZ7m0zPUDn85XEX2GTukHGRSepvi9Eig=
 github.com/klauspost/compress v1.17.4 h1:Ej5ixsIri7BrIjBkRZLTo6ghwrEtHFk7ijlczPW4fZ4=