Support disabling polling of pluggable SCM materials created via config repositories about gocd HOT 3 OPEN

Thanks for the sleuthing here. It does seem that this should be possible to address, and may be an oversight from the way things are modeled with autoUpdate being owned in config by the top level SCM along with values like id/name etc.

However there is some potential complexity/mess here to consider due to the way SCMs are de-duplicated which we'd need to reconcile to ensure the semantics are the same as for the non-pluggable materials (equally deterministics or non-deterministic).

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@chadlwilson Thanks for investigating this.
What do you think is the next step forward here?

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See the discussion at #11636 (comment) and below in particular. The extent of my thoughts are on the PR, and haven't got any further than that.

There's probably not really a next step without addressing the concerns on approach, which I'm not sure how to do right now (or put differently, have not had the time/energy/inclination to figure out).

The benefit appears somewhat small (given how long the config repo feature has been around and no-one highlighting this gap until now, that I can find!), and at the moment the effort/risk is relatively high, so we need to find a way to change that equation, which is rather difficult with the resources available.

TL;DR The current proposed approach in the PR has at least two problems

• can silently mutate configuration outside the config repository's scope, including materials that are not within scope of a given change, unintentionally affecting other pipelines.
• appears to mutate shared configuration objects in memory prior to a change being fully validated as safe to merge into the configuration, in a way that is not reverted/undone if the change later fails a validation constraint (this requires specific validation)

That's not really a solution I am comfortable with given the chance for unintended consequences that will be difficult for a user to reason about or debug.

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