ot::Dns::Name::AppendLabel() method

Encodes and appends a single name label to a message. The @p aLabel is assumed to contain a single name label as a C string (null-terminated). Unlike `AppendMultipleLabels()` which parses the label string and treats it as sequence of multiple (dot-separated) labels, this method always appends @p aLabel as a single whole label. This allows the label string to even contain dot '.' character, which, for example, is useful for "Service Instance Names" where portion is a user-friendly name and can contain dot characters.