disallow generated columns in distribution key

[yanwr1]

The generated columns are computed after distribution. if generated columns are used as distribution key, they will always use null values to compute the distribution key value, and it will cause wrong query results.

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[kainwen]

I am looking into this. Thanks for the PR.

[yanwr1]

Overall LGTM. Some comments:

1. please add comments in code also
2. please address review before pushing
3. do you try what if generated column as partition keys? (What is Postgres's behavior, and GPDB's)

Thanks.

generated columns can't be in partition keys both in postgres and GPDB.