Nested datastores - part 2 + disallow aliasing

[andre4i]

Description

ADO:7302

Unifying the creation APIs of the runtime and the channel collection. From a functional perspective, this is moving the logic to actually create/fetch datastores from the runtime to the channel collection. The runtime behavior would remain unchanged, as it delegates responsibility to the channel collection it owns.

This change also adds the concept of a level for datastores and disallows aliasing operations as currently they can lead to undefined (or incompatible) behavior.

Upcoming changes in subsequent PRs

• enable aliasing at levels below the runtime
• IFluidParentContext and IFluidDataStoreContext should eventually converge to the same signature
• (...)