Nested datastores - part 2

[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.

Upcoming changes in subsequent PRs

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