Add interface for NamingProvider and expand its use.

[RedX2501]

Use INamingProvider to also be able to change the class names from their
names in the XSD.

The idea is to be able to change the name of the whole class if necessary.
For example remove all underscores.

[codecov]

Codecov Report

Merging #106 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   97.13%   97.15%   +0.02%     
==========================================
  Files          17       17              
  Lines        2964     2988      +24     
==========================================
+ Hits         2879     2903      +24     
  Misses         85       85

Impacted Files	Coverage Δ
XmlSchemaClassGenerator/Generator.cs	75.55% <ø> (ø)	⬆️
XmlSchemaClassGenerator/GeneratorConfiguration.cs	95.23% <100%> (ø)	⬆️
XmlSchemaClassGenerator/NamingProvider.cs	100% <100%> (ø)	⬆️
XmlSchemaClassGenerator/ModelBuilder.cs	100% <100%> (ø)	⬆️

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[mganss]

Very good idea. In the default NamingProvider, can you extract the qualifiedName.Name.ToTitleCase(NamingScheme.PascalCase); into a single virtual method and make the other ones virtual as well, so people can you use them as a starting point?