feat(isthmus): mapping of positional scalar fns

[asuriy]

-Added mappings for STARTS_WITH, ENDS_WITH, CONTAINS and POSITION to FunctionMappings.java
-Added extra libraries to enable the included operators in SubstraitOperatorTable.java
-Added passing tests to StringFunctionTest.java including STARTSWITH, ENDSWITH, STRPOS and literals

[bestbeforetoday]

The additional SQL function names I suggested testing already work with the code you had previously since they are Calcite SQL aliases and resolve to the same Calcite Rel. For example, both the SQL ENDS_WITH and ENDSWITH result in the Calcite ENDS_WITH Rel node. This means that your mapping from Calcite ENDS_WITH to Substrait ends_with works for both SQL variants with no special handling in our code required. It does mean that when converting Substrait ends_with back to SQL you always get ENDS_WITH, even if the input SQL used ENDSWITH, but that is fine since they are equivalent. Calcite will deal with mapping its Rel representation to the correct SQL form for a target execution engine.

In summary, the only changes needed were the additions to the testing coverage. The implementation code was fine so the extra changes made can be removed. See the inline comments with suggestions that (if they look OK to you) you should be able to apply to take you back to the minimal changes required.