JSON version of display_indent()

[avantgardnerio]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

A significant portion of my time on #2885 was spent manually formatting subqueries like:

Projection: #SUM(lineitem.l_extendedprice) / Float64(7) AS avg_yearly
  Aggregate: groupBy=[[]], aggr=[[SUM(#lineitem.l_extendedprice)]]
    Filter: #part.p_brand = Utf8("Brand#23") AND #part.p_container = Utf8("MED BOX") AND #lineitem.l_quantity < (Subquery: Projection: Float64(0.2) * #AVG(lineitem.l_quantity)
  Aggregate: groupBy=[[]], aggr=[[AVG(#lineitem.l_quantity)]]
    Filter: #lineitem.l_partkey = #part.p_partkey
      TableScan: lineitem)
      Inner Join: #lineitem.l_partkey = #part.p_partkey
        TableScan: lineitem
        TableScan: part

into

Projection: #SUM(lineitem.l_extendedprice) / Float64(7) AS avg_yearly
  Aggregate: groupBy=[[]], aggr=[[SUM(#lineitem.l_extendedprice)]]
    Filter: #part.p_brand = Utf8("Brand#23") AND #part.p_container = Utf8("MED BOX") 
        AND #lineitem.l_quantity < (
            Subquery: Projection: Float64(0.2) * #AVG(lineitem.l_quantity)
                Aggregate: groupBy=[[]], aggr=[[AVG(#lineitem.l_quantity)]]
                    Filter: #lineitem.l_partkey = #part.p_partkey
                        TableScan: lineitem
        )
      Inner Join: #lineitem.l_partkey = #part.p_partkey
        TableScan: lineitem
        TableScan: part

because the indents aren't trustworthy when subqueries are taken into account.

Also, I've had a hard time writing unit tests for optimizers because building them programmatically is verbose and error-prone.

Describe the solution you'd like

If we were to create a visitor method like display_json(), and an equivalent LogicalPlan::from_json() then both problems would be solved fairly elegantly. As a bonus, plans would be readable and manipulable by common off the the shelf tools and readable by the uninitiated.

Describe alternatives you've considered

Fixing the indentation on the formatter, writing different types of tests, crying into my beer.

[avantgardnerio]

As a potential implementation option, we could use something like https://docs.serde.rs/serde_json/ with our existing visitor pattern. I know some folks are reticent to add dependencies though, so I thought I would open this up for discussion. I'm after the ends, the means I'm flexible on.

[avantgardnerio]

Thank you!!!