Cognito JSON Web Token (JWT) Verification

[EthanLozano]

The Verifying a JSON Web Token docs describe several steps for verifying id and access JWT's. The security of a developer's app depends on implementing these instructions correctly and without any vulnerabilities. It's easy to miss one of these steps. Missing a step negates most of the security that Cognito brings to an external application. There are a handful of instances online where someone incorrectly implements java JWT verification. It's a little unrealistic to expect every developer to implement this verification correctly.

Google has officially supported JWT verification for Google Sign-In within their "Using a Google API Client Library".

Please provide an officially supported implementation of JWT verification!

[shorea]

Closing per comments in the PR.