`runsc`: Add flag to control policy of sidecar binaries.

Intended as a stop-gap flag to allow users to fall back to embedded
binaries for some transition period, as a quick fix. This also forces
them to acknowledge the warning and the effects that depending on this
involves.

A future change will change the default behavior of this flag to
effectively `STRICT`. But this change itself has no functional effect yet.

PiperOrigin-RevId: 970171278