FEAT-#7607: Support pinning groupby objects in place.

[sfc-gh-mvashishtha]

Resolves #7607

[sfc-gh-joshi]

LGTM, just one high-level question I want to make sure of: do we want a groupby object's pin status to be permanently separate from that of its parent object, as it is in this PR?

df = pd.DataFrame(...).pin_backend()
groupby = df.groupby(...)  # should be pinned
df.unpin_backend(inplace=True)  # should not affect the pin status of the groupby object

[sfc-gh-mvashishtha]

do we want a groupby object's pin status to be permanently separate from that of its parent object, as it is in this PR?

That behavior makes more sense to me. I'm introducing inconsistency in this PR because we made the opposite choice for the indexer objects. Eventually, I prefer to align the indexer classes with the behavior here.