[FEATURE]: Add static_multiset::retrieve_outer

[sleeepyjack]

Is your feature request related to a problem? Please describe.

static_multiset requires an equivalent function to static_multimap::retrieve_outer.

Describe the solution you'd like

After #465 is finished, add a new API retrieve_outer and also add a template parameter to the kernel to switch between retrieve and retrieve_outer behavior.

Describe alternatives you've considered

Still an open discussion if this is the best approach to tackle this problem but we couldn't come up with a better solution so far.

Additional context

No response

[PointKernel]

This API is code-specific and should take probe hash and probe key equal as arguments.