Feature: add warehouse grammar and GUC variable

[roseduan]

Change logs

Since we are going to support storage/compute/catalog separation, we
need to add a warehouse logic to the system. Then we can use the
warehouse clause to specify the warehouse for a SQL query. A warehouse
is a group of segments, and we can create/alter/delete a warehouse.

This commit does the following things:

1. add the grammar for the warehouse clause.
2. add the GUC variable for the current warehouse.
3. add some fileter conditions for gp_segment_configuration table, since we
   need to filter out the segments that are not in the current warehouse.

Why are the changes needed?

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Does this PR introduce any user-facing change?

If yes, please clarify the previous behavior and the change this PR proposes.

How was this patch tested?

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