Non-ONVIF: Implement facet "fixed"

[victor-soloviev]

This page shows an example of fixed usage: https://docs.microsoft.com/en-us/previous-versions/dotnet/netframework-4.0/ms256061(v=vs.100)

fixed is not currently supported by the parser, due to ints absence in XSD files of ONVIF.

To implement fixed support:

• [ ] go to xsd-parser-rs/parser/xsd_element.rs and add facet "Fixed" into enum FacetType;
• [ ] go throug usages of FacetType::MaxInclusive to get idea of places where facets are processed in the code of the project;
• [ ] implement validation for Fixed similar to the way validation for MaxInclusive implemented.