[FEAT] [New Executor] [1/N] Move physical plan scheduler to new crate, misc. refactorings + drive-bys for new executor

[clarkzinzow]

This PR moves the physical plan scheduler to a new daft-scheduling crate in order to avoid cycle dependencies for the new executor, which depends on daft-plan. New dependency tree will be:

daft-plan   <--  daft-execution
       ^            ^
        \          /
      daft-scheduling 

This PR also contains misc. refactorings and drive-bys for follow-up new executor PRs, and will be the bottom of a stack of PRs.

[samster25]

LGTM! Nothing controversial about the changes here!