Support [NO] INDENT option in XMLSERIALIZE().

This adds the ability to pretty-print XML documents ... according to
libxml's somewhat idiosyncratic notions of what's pretty, anyway.
One notable divergence from a strict reading of the spec is that
libxml is willing to collapse empty nodes "<node></node>" to just
"<node/>", whereas SQL and the underlying XML spec say that this
option should only result in whitespace tweaks.  Nonetheless,
it seems close enough to justify using the SQL-standard syntax.

Jim Jones, reviewed by Peter Smith and myself

Discussion: https://postgr.es/m/2f5df461-dad8-6d7d-4568-08e10608a69b@uni-muenster.de

src/include/nodes/parsenodes.h

@@ -840,6 +840,7 @@ typedef struct XmlSerialize
 	XmlOptionType xmloption;	/* DOCUMENT or CONTENT */
 	Node	   *expr;
 	TypeName   *typeName;
+	bool		indent;			/* [NO] INDENT */
 	int			location;		/* token location, or -1 if unknown */
 } XmlSerialize;
 

src/include/nodes/primnodes.h

@@ -1464,7 +1464,7 @@ typedef enum XmlExprOp
 	IS_XMLPARSE,				/* XMLPARSE(text, is_doc, preserve_ws) */
 	IS_XMLPI,					/* XMLPI(name [, args]) */
 	IS_XMLROOT,					/* XMLROOT(xml, version, standalone) */
-	IS_XMLSERIALIZE,			/* XMLSERIALIZE(is_document, xmlval) */
+	IS_XMLSERIALIZE,			/* XMLSERIALIZE(is_document, xmlval, indent) */
 	IS_DOCUMENT					/* xmlval IS DOCUMENT */
 } XmlExprOp;
 
@@ -1489,6 +1489,8 @@ typedef struct XmlExpr
 	List	   *args;
 	/* DOCUMENT or CONTENT */
 	XmlOptionType xmloption pg_node_attr(query_jumble_ignore);
+	/* INDENT option for XMLSERIALIZE */
+	bool		indent;
 	/* target type/typmod for XMLSERIALIZE */
 	Oid			type pg_node_attr(query_jumble_ignore);
 	int32		typmod pg_node_attr(query_jumble_ignore);