Allow using dim expressions as accessors

[philippjfr]

In a recent PR we added support for dim expressions which can call arbitrary functions on the datastructure they will be applied to. This PR generalizes this mechanism to allow not just calling methods but also using accessors on the underlying object, e.g. to use datetime accessors on a pandas Series:

dim_expr = hv.dim('dates').dt.week
ds = hv.Dataset(pd.date_range('2012', '2013'), 'dates', datatype=['dataframe'])
ds.transform(week=dim_expr).data

[github-actions]

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