[VOTE] Add covering_fields to IndexMetadata

[vivek-bharathan]

Formal vote following this discussion.
Implemented in this PR

Adds a new field to IndexMetadata: repeated int32 covering_fields = 11;

An index today records only the columns it is keyed on (fields). This new field records the columns whose values are stored alongside the index's own data, so a query whose projection is satisfied by them can be answered from the index without a take against the base table.

[Xuanwo]

+1 binding

[jackye1995]

Can we call it covering_fields instead? I don't know why I proposed the word included originally tbh... but given this is for covering index, covering fields seem like a more natural fit?

Outside that, I am also +1

[vivek-bharathan]

Changed it to covering_fields

[wjones127]

+1

[westonpace]

+1