Add support for base64url regex validator

[TimoGlastra]

It would be great if support for a base64url regex validator could be added in addition to base64. We heavily rely on base64url over base64.

A very simple/naive regex would be:

[a-zA-Z0-9_-]+

But looking at the regex for base64 it may needs some extra parts.

[fabian-hiller]

Feel free to create a PR for a base64Url action. In the meantime you can fallback to regex or check.

[EltonLobo07]

@TimoGlastra Are you planning on creating a PR for this action, or would you prefer someone else to do it? I can get it done if you are not planning on creating a PR.