External probe: Add ability to retrieve external probe binary (or script/source code) from a remote location (Git/GCS/S3)

[manugarg]

Copied from google/cloudprober#25, originally filed by @manugarg.

I am not super sure about it, but it may allow for quick prototyping.

[manugarg]

@DazWilkin's comment:

Having been initially drawn to deploying Cloudprober (as a container) on Kubernetes, I propose we consider a model in which external probes are (also) deployed as containers.

For Kubernetes, this would then enable delivering external probes as sidecars but this approach appears to provide more general benefits:

• leveraging container registries for hosting external probes (rather than GCS|S3);
• unifying deployments; and
• perhaps consider network (simple http) rather than stdout/pipe-based integration?

The latter could make external probes just a special case of http (or perhaps udp) probes.

[manugarg]

Other interesting options:

• We can fetch Python, Go, Shell code from Github (git-sync) as well, and then run that code through local Python interpreter, Go compiler, and Shell interpreter respectively.

There are a couple of changes required here:

• Provide a way to get files from Git.
• Provide a way to configure external probe to retrieve such code, and run it differently.