feat(isthmus): add method converting SQL to POJO Plan

[nielspardon]

I noticed that #432 could benefit from this refactoring.

BREAKING CHANGE: deprecates SqlToSubstrait.execute()

• adds a new convert() method to SqlToSubstrait which converts a SQL statements string to a POJO Plan instead of a proto Plan
• deprecates the execute() method in SqlToSubstrait which converts a SQL statements string to a proto Plan
• changes any uses of execute() in the code to convert()
• removes the @VisibleForTesting annotation from SqlToSubstrait.createSqlToRelConverter() and SqlToSubstrait.getBestExpRelRoot() since they are not actually used in testing and makes them protected instead of package private
• adds @VisibleForTesting annotation to SqlToSubstrait.sqlToRelNode() which is actually still used in testing
• changes the TpcdsQueryTest and TpchQueryTest to test POJO to SQL and PROTO to SQL separately since I noticed that TPC-DS query 67 only fails for PROTO to SQL but started working to POJO to SQL

[vbarua]

Changes look good to me. This in inpsired me to push through my own changes extending these. I'll open a PR for those shortly.

Also, I don't believe this a breaking change as you've only deprecated the method, not removed it.

[vbarua]

Taking this even further in #474