feat: Updates AccessListAPIKey with recent changes in CFN adding new read only property

Proposed changes

Jira ticket: CLOUDP-252774

Reflects changes coming from https://github.com/mongodb/mongodbatlas-cloudformation-resources/pull/1058, adding a readonly property Entry which is used for composing the primary id of the resource.

Added README.md file that was missing for this resource.

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] This change requires a documentation update

[x] I have signed the MongoDB CLA
[ ] I have added tests that prove my fix is effective or that my feature works
[x] I have added any necessary documentation (if appropriate)
[x] I have run make fmt and formatted my code
[ ] I have tested the CDK constructor in a CFN stack. See TESTING.md
[x] If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.