Feature request: From/To Base64 in AWS format

**Is your feature request related to a problem? Please describe.**
AWS signed urls for instance use this alphabet for base64: `A-Za-z0-9-~_` as documented [here](https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/private-content-creating-signed-url-custom-policy.html#w791aac17c19c17c21c13b5b8b3)

**Describe the solution you'd like**
Add this alphabet to the list of available alphabets in base64 encoding/decoding

Start by locating CyberChef's Base64 encoding and decoding operations and the list of available alphabets. Add the AWS alphabet described in the issue, then verify that both encoding and decoding support the A-Za-z0-9-~_ characters and that the existing Base64 behavior remains intact.

Written by the indexing model from the issue text.