Support encoding of strings to base64

[sbuettner]

Is your feature request related to a problem? Please describe.

Some APIs require data to be sent as base64 encoded strings. Even for text values like textfile contents. It would therefore be handy to be able to encode strings right inside the feel engine via a OOTB function that supports strings.

Describe the solution you'd like

We should support an encode base64 function that accepts a string as the input and returns a base64 encoded string as the output.

Related issues

camunda/connectors#2023