add ability to view debug logs for a program in the viewer app

[demmer]

A user running a juttle program through juttle-engine should be able to opt in to see the internal debug logs generated by their program. This can be helpful to figure out whether backend queries are returning any data and to help debug what is happening in the runtime when things are not working as expected.

This requires support in both juttle-service and juttle-viewer:

• juttle-viewer/#7
• juttle-service/#42