XMLTYPE: methods - final step

XMLTYPE: step 3 — remaining methods after step 1 (MDEV-36261) and step 2 (MDEV-38690).

Step 1 delivered the SQL syntax `xmltype_expr.method()` plus the lightweight getters:
getNamespace, getNumberVal, getRootElement, getSchemaURL, getStringVal

Step 2 delivered the data-access methods:
existsNode, extract, getClobVal, getBlobVal (or getBlobVal as synonym for getClobVal)

This step 3 ticket delivers the remaining Oracle 19 surface:

Constructors:
createXML, createSchemaBasedXML, createNonSchemaBasedXML

Schema-related member functions:
isSchemaBased, isSchemaValid, isSchemaValidated, isFragment

Schema-related member procedures:
schemaValidate, setSchemaValidated

Other:
toObject, transform

Reference: https://docs.oracle.com/en/database/oracle/oracle-database/19/arpls/XMLTYPE.html

Closing this ticket completes XMLTYPE method support and unblocks MDEV-20658.