Summary:
Added integer field to placement zone which represents zone priority.
These priorities should be non-negative and should form contiguous sequence.
When we are updating placement info in db, we will add additional call set_preferred_zones
if we see that placement is using new priorities.
All the current placements are using old api (affinitized zones in placement modification request)

Test Plan:
sbt test
local provider test

Reviewers: nsingh, anijhawan

Reviewed By: nsingh

Subscribers: yugaware

Differential Revision: https://phorge.dev.yugabyte.com/D45860