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Add support for the method .query() for XML data type (#4786)

Currently, Babelfish supports .exist() and .value() XML methods but .query() was unsupported and threw an error.
With this change, .query(xpath) is now supported and returns XML results (for XPath 1.0 compatible queries).

The implementation follows the same 3-stage architecture like .exist() and .value():

ANTLR rewriting: @x.query('/path') is rewritten to sys.bbf_xmlquery('/path', @x)
Grammar: BBF_XMLQUERY is mapped in gram-tsql-rule.y
PL/pgSQL function sys.bbf_xmlquery(): validates the input type, checks QUOTED_IDENTIFIER, calls PostgreSQL's
xpath(), and returns the result as XML

Task: BABEL-5224
Signed-off-by: Japleen Kaur <amjj@amazon.com>

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