Ability to track status of optimize api

[ppf2]

Optimize is an intensive process and can run for many hours. It will be nice to provide a way to track its status (esp. when we switch to a sync operation in the future: #9640).

This is likely dependent on task management:
#6914

[kjelle]

+1

E.g. im optimizing a 24shard index from 1127 segments down to max_num_segments=1... it would be nice to see that it is actually moving forward, estimate how long it will take etc.

[clintongormley]

Closing in favour of #15117