Enable tests for generating schema with StringEnumConverter.

Enable an old test of generating a schema using the JsonSchemaGenerator
for an enum with the StringEnumConverter. Modify it to no longer expect
the output enum values to be camelCased, but add a new test for this
scenario
Set Required-property for custom converter schemas.

Set the Required-property for schemas generated by custom converters if
needed.
Make StringEnumConverter GetSchema camelCase-aware.

In StringEnumConverter, change the enum value names in generated schemas
to camelCase if enabled.