runners: refactoring to simplify & future-proof

[iurii-ssv]

This PR refactors a bunch of code around Runner(s) (and BaseRunner in particular):

• rearranges some code to make runners a bit more flexible (eg. so we can implement a Runner that can execute a bunch of things in parallel)
• simplifies & clarifies some things

It is a subset of functionality I've implemented while working on pre-confirmations (initial draft) - I'm pulling this refactoring out into separate PR here because:

• preconfs: preconf-commitment duty (initial draft) #2063 might never be merged (if we decide to never finalize the pre-confirmations design it was targeting) ... while the refactoring itself is something we'll likely need eventually to evolve Runner(s) codebase
• preconfs: preconf-commitment duty (initial draft) #2063 grown large as it is - so merging it separately will simplify reviews (and also I won't need to keep it relevant by constantly rebasing preconfs: preconf-commitment duty (initial draft) #2063 itself)

Also closes #2311

Before merging:

• resolve TODOs
• test on stage

[codecov]

Codecov Report

❌ Patch coverage is 71.05263% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.3%. Comparing base (164e25c) to head (d185aba).
⚠️ Report is 1 commits behind head on stage.

Files with missing lines	Patch %	Lines
operator/validator/controller.go	68.7%	10 Missing ⚠️
operator/validator/task_executor.go	83.3%	1 Missing ⚠️

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[iurii-ssv]

Force-pushing to make sure every commit in this PR is gpg-signed (otherwise we cannot merge it)

[oleg-ssvlabs]

good stuff. I left few questions

[y0sher]

Great job, code looks much cleaner and a lot of tech debt was resolved 🔥