Instead of a bizare mechanism of signals back to the manager
object that used to be required because of the user/system settings
split, let each place that needs secrets request those secrets
itself. This flattens the secrets request process a ton and
the code flow significantly.
Previously the get secrets flow was something like this:
nm_act_request_get_secrets ()
nm_secrets_provider_interface_get_secrets ()
emits manager-get-secrets signal
provider_get_secerts ()
system_get_secrets ()
system_get_secrets_idle_cb ()
nm_sysconfig_connection_get_secrets ()
system_get_secrets_reply_cb ()
nm_secrets_provider_interface_get_secrets_result ()
signal failure or success
now instead we do something like this:
nm_agent_manager_get_secrets ()
nm_agent_manager_get_secrets ()
request_start_secrets ()
nm_sysconfig_connection_get_secrets ()
return failure or success to callback
