feat(common): add Node.find_below() methods to exclude the root node from filtering

[kszucs]

Currently we are unable to locate certain values under a relation while applying a value filter because the root node gets filtered out, like:

relation.find(pattern, filter=ops.Value)

Used in practice at #8825

for child in _.__children__ + _.parent.__children__:
    if child.find((Window, ops.ExistsSubquery, ops.InSubquery, ops.Unnest), filter=ops.Value):
        ...

Which can be simplified to _.find_below() or _.parent.find_below() after this feature.

I am not that satisfied with the name, but marked as experimental so we can change it anytime.
This feature will also help with pretty printing the dereference mapping without traversing the dictionary keys in _flatten_collections().

[cpcloud]

Seems fine, just wondering what the plans are for find_below.