Add option for configurable post-start hooks

[dominiklohmann]

📔 Description

The new option vast.start.commands allows for specifying an ordered list of VAST commands that run inside the node after successful startup. This is useful for declarative deployments that run sources or matchers (via the proprietary Live Matching plugin) in the node.

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/vast, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

Try locally. This can hardly be tested, sadly, since we do not have unit tests for the start command and cannot reliably test remote sources in the integration test framework. I played around with the following:

vast:
  start:
    hooks:
      # Requires PCAP plugin
      - spawn source pcap -i en0
      # Requires NetFlow  plugin
      - spawn source netflow -i :4739/udp
      # Requires Live Matching plugin
      - matcher start --match-types=address ips

[mavam]

I think "hook" is too generic. We can be more specific here: it's really just a sequence of commands and we can call them what they are.

[mavam]

This seems like a good middleground to get experience with post-hoc configuration of VAST using commands with side effects.

That said, I cringe a bit in that it could be more declarative to fit the surrounding configuration. I don't think that's a dealbreaker though; this is better than the current state.

@tobim @lava any objections from your side?

[dominiklohmann]

I received @lava's approval to go ahead with this design in a call earlier.