Suggestion: rename fields

[rmmariano]

I would like to suggest renaming some fields:

(1) In the collections table, rename extent to spatial or extent_spatial to make it more similar to the STAC specification, because there is both the extent spatial andextent temporal.

(2) In the collections anditems tables, rename the created andupdated fields to created_at andupdated_at to make the type of fields more explicit, because leaving only created and updated , it sounds they are flags (i.e. boolean fields), while the suffix _at makes them explicit that they are dates.

[gqueiroz]

We will named this collection: spatial_extent