Mark all of the ProxiedInRepeated and ProxiedInMapValue traits as hidden and with `Private` token as first type.

These methods are intended to be implementation detail; application code is
disallowed from implementing these traits (by our 'SealedInternal trait') and
should also never be called by users.

The `Private` token being an argument actually truly prevents users from
calling them in blazel (where its defined in a build target that gencode uses
but that application code isn't allowed to), in Cargo its only a "Sign not a
cop" but we don't subscribe to semver if someone ignores the sign.

PiperOrigin-RevId: 859256243