docs: add implementation checklist for OpenID4VP specifications

[coplat]

Description of change

Add a checklist outlining all the specifications which form part of OpenID4VP as a means of tracking our implementation progress.

Links to any relevant issues

Fixes issue #88

How the change has been tested

Describe the tests that you ran to verify your changes.
Make sure to provide instructions for the maintainer as well as any relevant configurations.

Definition of Done checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes