SOR: implement polling to retrieve the current model version

[pgayvallet]

Part of #150312

Depends on #152219

In managed mode, the SOR will have to retrieve the informations about the current model version from the index's meta to know which version it should convert to for writes.

The scope of this issue is only to implement the polling mechanism, not to use it

Technically, we need the polling to be performed only once (one actor) for all of the instantiated repositories. So we will probably want to introduce the concept of SavedObjectModelStatusProvider (or any better name) that will be shared between all the repositories (and probably created/managed by the top level service)

questions

• are we safe to stop polling once the index version matches the app version ? what about rollbacks ?

[pgayvallet]

This is not going to be necessary for the MVP.

Keeping the issue open though, as it may come back later.

[rayafratkina]

Closing for now - we will recreate or reopen if/when needed