Add generators project for source generation

[nickfloyd]

Fixes: #2534

This PR intends to add an iteration 0 for code generation. The initial generator was sourced from the work @Saalvage did when adding the AsyncPaginationExtensions.

This PR:

• Adds the generator project
• Adds the first generator - AsyncPaginationExtensions (initial approach - future iterations will dive into using things such as Rosyln and source templates
• Adds a readme explaining how to use this project and why we are doing it
• Regenerates the extensions - there are deltas due to white space, some template modification, and ordinality of the generated implementations

Note: The existing test coverage for the extensions is still intact, and I intentionally left testing off of the generations project because:

1. It was redundant to test the generated extensions
2. This project is effectively a prototype for code generation and is proven through compiling the source it has generated

[nickfloyd]

release_notes: Adds generators project and AsyncPaginationExtension generator