core: better errors and more fields for io.systemd.Unit.StartTransient (#42161)

core: add User,Group,SupplementaryGroups,Nice to varlink
 Unit.StartTransient

This commit adds more writable fields to the
io.systemd.Unit.StartTransient
varlink method. With this its possible to set:
User,Group,SupplementaryGroups,Nice values.

Plus tests for them.

---

core: report unsupported service fields in varlink calls

Just like for the unsupported/bad exec_fields we should show
a message about what field is bad for service parameters. This
commit adds it using the same pattern. The JSON parser works in
fail-fast mode so we only display the first bad field (and
it depends on the parser what it finds first).