Add optional preheader and AMP fields

[turnerdev]

Add support for preheader and amp_html optional fields for template and render operations

Description

Add preheader and amp_html to TemplateVersion to support create, read and update operations.
Add preheader and amp_html to RenderTemplateResponse.

Motivation and Context

Parity with the current Sendwithus API

How Has This Been Tested?

• All new and currently passing NUnit tests pass (some unrelated test are currently failing).
• Created a test consumer to verify all affected operations work as expected against test and production environments.
• amp_html not yet GA, omitted from test cases.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.