Implement a new indexing strategy on streams that indexes arbitrary properties

[albe]

Following the concept of "Lazy event sourcing" from https://www.youtube.com/watch?v=aqv8d1pjmU8 it might be a nice feature to be able to arbitrarily index events, such that models can be made up lazily on the go by just querying according events. The current stream indexing method (one index per property value combination, e.g. for every "userId") would lead to too many indexes being created potentially. Drawback is that those indexes would become much more complex and likely would need to implement a B-tree like structure to quickly find elements.