Enable Canonical XML (C14N) for MaterialX i/o

[ashwinbhat]

MaterialX XML i/o should provide options to be able to write Canonical XML. Here are a few examples https://www.w3.org/TR/xml-c14n11/#Examples
PugiXML does not support built-in support for writing Canonical XML and will need custom code to support this.

Canonical XML 1.0

• disallow non-escaped characters
• fix namespace "::" usage as it's not equivalent to an XML namespace, or "fake" XML namespace support
• remove reliance on custom comments esp. for definition documentation.
• add real node
• remove custom newline support

Canonical XML 1.1

• commented vs uncommented variant
• more strict on formatting and ordering
• normalization occurs.

This is a sub issue for #2956

[lrosenthol]

Why do you need Canonical XML over standard XML?