minor: Refactor PhysicalPlanner::default() to avoid duplicate code

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

As we add more expressions from datafusion-spark we would need to update two identical code blocks, so this PR refactors them to avoid duplication.

What changes are included in this PR?

How are these changes tested?

[andygrove]

Note that Self::new will register the UDF:

    pub fn new(session_ctx: Arc<SessionContext>) -> Self {
        // register UDFs from datafusion-spark crate
        session_ctx.register_udf(ScalarUDF::new_from_impl(SparkExpm1::default()));
        Self {
            exec_context_id: TEST_EXEC_CONTEXT_ID,
            session_ctx,
        }
    }

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.39%. Comparing base (f09f8af) to head (2566439).
Report is 233 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1821      +/-   ##
============================================
+ Coverage     56.12%   59.39%   +3.27%     
- Complexity      976     1151     +175     
============================================
  Files           119      129      +10     
  Lines         11743    12628     +885     
  Branches       2251     2368     +117     
============================================
+ Hits           6591     7501     +910     
+ Misses         4012     3919      -93     
- Partials       1140     1208      +68     

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[comphead]

nice clean @andygrove