[VOTE] Add has_null_bitmap feature flag in zone map details

[westonpace]

We recently made a change to zone maps so that they record the null bitmap in addition to stats. This makes it very easy to satisfy IS NULL and IS NOT NULL queries. However, at the time, we didn't update the details.

I'd like to update the details to indicate when this null bitmap is present. This will help us with planning time decisions around whether we should use the index or not. Currently we use the index for IS NULL but not for IS NOT NULL because we assume the latter will be inefficient as zone maps are an inexact index. By adding this flag we will know when a zone map is new (and thus efficient) so we can make the right choice at planning time.

The flag is backwards and forwards compatible.

Backwards: If the flag is not present we assume there is no null bitmap.
Forwards: Unknown flags are ignored during details deserialization. The null bitmap can be safely ignored by a reader.

[westonpace]

Implementation here: #8088

[wjones127]

+1

[BubbleCal]

+1

[Xuanwo]

+1

[westonpace]

The vote passes.