Add support for serializing additional types

[Thomas Clark]

Is your feature request related to a problem? Please describe.
This package currently only supports serializing String types and any custom types that are annotated with XmlSerializable().

Describe the solution you'd like
As a developer I would like this package to serialize types such as int and bool so that I do not have to implement that logic.

Describe alternatives you've considered
The current workaround is for the consuming application to implement logic to convert String types to other common types.

Additional context
This change will involve refactoring the XmlSerializableGenerator and creating SerializerGenerators that abstract the logic.