[FEA] Unify distinct_count column/table APIs.

[bdice]

Is your feature request related to a problem? Please describe.
While reviewing #10030, I found that the column and table algorithms for distinct_count have completely different flags for null and NaN handling. The column API has null_policy (include/exclude) and nan_policy (NaN is/isn't null), while the table API has null_equality (nulls are equal/unequal).

This also applies to unordered_distinct_count, introduced in #10030.

Describe the solution you'd like
The distinct count APIs for column/table should use the same flags (meaning that all three flags should probably be available to both APIs). This would also allow the column API to be a pass-through implementation of the table API, with a table composed of only that column, rather than having two implementations (table, column).

[github-actions]

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[github-actions]

This issue has been labeled inactive-90d due to no recent activity in the past 90 days. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed.