[C++][Acero] Create BackpressureSource and BackpressureCombiner

Some ExecNodes require handling multiple sources of backpressure. I propose providing genalized `BackpressureCombiner` that will allow handling of multiple `BackpressureCombiner::Source`s.
Source can be connected to BackpressureCombiners in many to many pattern. In ExecNode BackpressureCombiner would handle one input of exec node. Source take backpressure from output and any additional internal queues can provide extra Sources. Connections from Sources to Combiner would be created at runtime during init.
Example configurations:
- 1 x Source-> N x BackpressureCombiner - UnionNode case - propagate output backpressure to all inputs
- N x Source-> 1 x BackpressureCombiner - TeeNode case - propagate output backpressure and writer backpressure to input

Additionally the Source-BackpressureCombiner connections can be defined as strong or weak.
- Strong connection will always propagate pause upstream.
- Weak connection will propagate pause only if _all_ sources with _weak_ connections are paused.

This should allow to cover all possible backpressure propagation schemes - this will be especially useful with pipe concept.