Revise pipeline operator API

[jachris]

This PR makes some changes to the implementation of RFC-001 (#3004). The main goal is to simplify operator definitions and internal execution details.

• The instantiation state is now only stored by the instantiated generator.
• There is no more physical_operator/logical_operator separation in the code base, because we have only one intermediate representation for both (though we can still use these terms in explanations).
• Operators can be copied without a parse(to_string()) round-trip.
• Schema-specific dispatching is an implementation detail of the operator, but we provide a class that makes this convenient. This drastically simplifies the execution engine.
• Polymorphic operators are now possible. For instance, head could be made to work on both event and byte streams.
• The empty pipeline is therefore now also correctly typed, same as pass.

Pipeline serialization is left for a follow-up PR.

[dominiklohmann]

I went through the code; I'd like to take a second look after the feedback is incorporated.

[dominiklohmann]

Great work! Now to rebase all the other PRs.... 😅