[trel-dnssd] prioritize wider-scope IPv6 address of TREL peers

[abtink]

This commit updates TrelDnssd to select the numerically smallest IPv6 address when a peer advertises multiple IPv6 addresses. This prioritizes globally-unique addresses (GUA) over unique-local addresses (ULA) with fc00::/7 prefix, followed by link-local addresses (fe80::/10).

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Should address #2116.

[librasungirl]

👍

[superwhd]

LGTM regarding the logic of selecting address.

However, I think this alone may not be able to resolve the issue. Today in OTBR the mDNS resolvers won't return multiple addresses in one callback. When a host has multiple addresses, mDNS resolvers will trigger a callback for every address.

To fully fix the issue, we may also need to enhance the mDNS resolvers to make it return all resolved addresses at one time.

[abtink]

To fully fix the issue, we may also need to enhance the mDNS resolvers to make it return all resolved addresses at one time.

@superwhd, I think Publisher behavior to return the first address during Broswe/ServiceDiscovery may be fine and was intended behavior.

But there is a missing behavior (never implemented?) in trel_dnssd platform code which is mentioned/required in trel.h API documentation and I expect would address this. Upon discovery of a peer service, the platform should start an ongoing DNS-SD query for AAA records on the hostname of service (which should then discover all addresses on host):
trel.h

 * events are not guaranteed, however. When a TREL service instance is discovered, a new ongoing DNS-SD query for an
 * AAAA record should be started on the hostname indicated in the SRV record of the discovered instance. If multiple
 * host IPv6 addressees are discovered for a peer, one with highest scope among all addresses MUST be reported (if
 * there are multiple address at same scope, one must be selected randomly).

This would address cases where the host addresses get changed (which wont work today).