[Explore] Implement k6 probe type

[manugarg]

With the evolution of the browser probe type (https://github.com/orgs/cloudprober/projects/6), I am beginning to think about leveraging the work that's been done with respect to:

• Running external processes (https://github.com/cloudprober/cloudprober/pull/854)
• Parsing different output format (https://github.com/cloudprober/cloudprober/pull/841)

to run more external command based probes like k6, for example. I am guessing the delta between browser probe type, which is essentially a playwright probe type, and k6 probe type will not be much. We'll probably need to write a custom output module for k6 or parse json output from k6.