NMSockAddrEndpoint is an immutable structure that contains the endpoint
string of a service. It also includes the (naive) parsing of the host and
port/service parts.

This will be used for the endpoint of WireGuard's peers. But since endpoints
are not something specific to WireGuard, give it a general name (and
purpose) independent from WireGuard.

Essentially, this structure takes a string in a manner that libnm
understands, and uses it for node and service arguments for
getaddrinfo().

NMSockAddrEndpoint allows to have endpoints that are not parsable into
a host and port part. That is useful because our settings need to be
able to hold invalid values. That is for forward compatibility (server
sends a new endpoint format) and for better error handling (have
invalid settings that can be constructed without loss, but fail later
during the NMSetting:verify() step).