docs(awscognito): add custom claims example

[nodomain]

After struggling with my custom department claim and getting helped in https://discourse.jupyter.org/t/genericauthenticator-with-cognito-how-to-check-for-department-match/18105/1 I wanted to give back to the community.

• Add / update documentation
• Add tests

[welcome]

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[consideRatio]

@nodomain sorry for not being able to review and get this merged quicker! We have used .rst based documentation in the past, but transitioned to markdown now (MyST markdown) via #863.

Are you willing to update this PR with changes to the markdown file instead? Ping me and I'll get this merged quickly if you do!