eventstream, support BASE64 -> text transformations

[svelde]

I notice that incoming messages that are BASE64 encoded are not automatically translated to JSON.

Please make it possible to let users either transform BASE64 to text (or actually JSON) via a new transformation option.

Or, provide automatic BASE64 transformation in the sources via eg. checkboxes so users have more control over the inner working of the Eventstream sources.

[svelde]

This way, the Eventream flow is still applicable to the body of (now decoded) incoming messages so eg. filtering or enrichment or grouping can be done in the values in the (previously encoded) body.