Consider GIN index on metastore for tags field

[r-zenine]

Describe the solution you'd like
It would be nice to benchmark the GIN index in the postgresql metastore in order to speed up queries related to tag pruning.
It could improve performance for huge datasets with many splits and a lot of tags.

[fmassot]

Use case where it can boost search query latency:

• multi indexes (> 1000 indexes) search
• search by tag, which is present only on 1 index

With a table containing 300k split, most of the time of a search query is spent on list_splits.

To make the GIN index, we probably need to use the array operator, see this SO post.