Add useSplitManager hook

[EmilianoSanchez]

React SDK

What did you accomplish?

• Add useSplitManager hook to replace useManager. Similar to useSplitClient and useSplitTreatments for useClient and useTreatments respectively, it returns an object with the Split context together with the manager instance.

const { manager, isReady, ... } = useSplitManager() // new hook

const manager = useManager() // deprecated

How do we test the changes introduced in this PR?

• Unit tests

Extra Notes