[Integration]Adding the initial Integration resource

[wbkang]

Issue #, if available: https://github.com/aws-cloudformation/aws-cloudformation-resource-providers-rds/issues/485

Description of changes:

This is an initial version of the Integration resource. It does have good unit test coverage, and I've done some very minimal testing but contract testing is not passing yet.

I'd like to have this reviewed and committed so we can make smaller PRs in the future.

The implementation was co-authored by @khebul (recorded in the commit as well).

[khebul]

Can you add the maven-verify workflow to github actions for the resource?

[wbkang]

I've added maven-verify workflow in the latest update.