libnm,core: move _nm_connection_for_each_secret() from core to libnm-core
_nm_connection_for_each_secret() (formerly for_each_secret()) and _nm_connection_find_secret() (formerly find_secret()) operate on a GVariant of secrets. For that, they implement certain assumptions of how to handle secrets. For example, it must special-case VPN settings, because there is no generic abstraction to handle regular secret and VPN secrets the same. Such special casing should only be done in libnm-core, at one place. Move the code to libnm-core as internal API.
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