Added the first version of PostgreSQL async projections

[oskardudycz]

1. It can support both generic handling and async projection abstractions.
2. It should be possible to plug it into EventStoreDB consumer, but that'll be tested in the follow-up PR. The key for that was allowed to pass connection options for the processor and using them as a fallback if the context got from the consumer doesn't have it. This will be useful also if you'd like to store read models in a different database than consumer code.
3. Simplified ProjectionDefinition type and merged TypedProjectionDefinition into. That was made possible thanks to introducing AnyEvent type and enabling contravariance.