Add base64URL as encode / decode format

[gramian]

The encode and decode SQL functions only support base64 encoding. The problem with that is that base64 uses characters that have meaning in URLs, ie + and /. A derived encoding base64URL replaces + with -, and / with _, which are both characters without need for url encoding. Thus I would like to propose to add base64URL as a format.

I think this shouldn't be too hard, as in the resulting encoding or passed encoded string the characters in question would just have to be replaced before decoding or after encoding with base64.

[lvca]

Thanks @gramian to support this on #1019. Closing this discussion.