feat(isthmus): add support for scalar subqueries

[nielspardon]

Currently, the Substrait to Calcite mappings to not support scalar subqueries like the follow SQL example:

select 
  n_name 
from 
  tpch.tiny.nation 
where 
  n_regionkey = (
    select r_regionkey from tpch.tiny.region where r_name = 'EUROPE')

This PR adds support for mapping scalar subqueries from Substrait into Calcite and adds a test case showing that it works now.

Additionally:

• it changes RelCreator to be created with a list of CREATE SQL statements to initialize the schema of the RelBuilder
• the previous change requires RelCreator to extend SqlConverterBase so that we can reuse the registerCreateTables() method
• in SubstraitExpressionConverterTest it gets the ExpressionRexConverter from SubstraitRelNodeConverter in order to make sure the ExpressionRexConverter is properly set up to parse sub queries using SubstraitRelNodeConverter

[vbarua]

The conversion code looks good overall. I left some comments that will hopefully simplify the testing of this a little bit.

[vbarua]

Left one comment with a simplification for the NamedStruct gatherer, but otherwise looks good to me.