Split precompilation and runtime behavior in different traits

[jprendes]

Currently the precompilation behavior of a shim is defined in the same Engine trait where the runtime behavior is specified.

The two behaviors are linked, in the sense that they need to be compatible. But they are also independent, as precompilation needs to run in the shim process while the runtime runs out of process.

I propose we split them in two different traits, where the Precompiler trait could be an associated type to the Engine trait.

[Mossaka]

We talked about this issue in detail in our last community call. Please check the notes here: https://docs.google.com/document/d/1aOJ-O7fgMyRowHD0kOoA2Z_4d19NyAvvdqOkZO3Su_M/edit?tab=t.0

[fibonacci1729]

+1; had the same thought while working on #792