Move ExecutionPlan and related structures into datafusion-execution crate

[alamb]

Which issue does this PR close?

Part of #1754

Rationale for this change

I am working on extracting datafusion/core/src/physical_plan and datafusion/core/src/physical_optimizer and datafusion/core/src/datasource to their own crates.

However, they all depend on ExecutionPlan

What changes are included in this PR?

Hoist ExecutionPlan and related types / structs into datafusion_execution

I envision the dependency tree looking like:

datafusion-core -----------> datafusion_datasource ---------------------> datafusion_execution
\------datafusion_physical_optimizer --> datafusion_physical_plan --/

Are these changes tested?

existing tests

Are there any user-facing changes?

I don't think so -- I left pub use's around to avoid breaking API changes

[alamb]

could also be convinced this is not a good idea (leaving all the code in datafusion_physical_plan) if possible

[alamb]

I am still thinking about if this is a good idea -- there is a certain benefit to keeping ExecutionPlan in the physical_plan module which has most of the structures implementing ExecutionPlan

[alamb]

I think it will be very hard to decouple datasource from physical_plan and thus I am going to postpone this PR

Instead I will proceed via #6516