Create search index assignments for notes

What does this MR do and why?

This MR introduces configurable "index assignments" for a root-level namespace. Previously the index was hard-coded for all namespaces. This will allow us to use partitioning by hashing the root-level namespace ID and using that to organize data into dedicated indices. (See epic: &9919)

Issue: #392337 (closed)

Additionally closes #393184 (closed)

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