Add flag to not load any default pipeline

[timotheeguerin]

Add --no-default-pipeline flag to disable loading rest of the pipeline.

TBD if we want this:

• one problem is using this flag would make it incompatible to run a plugin that set it with other plugin that expect it not set.

Really what we would have wanted here is there is no default pipeline and a plugin ask for what it needs and the pipeline is generated up to that point. But don't think we can change that today.

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[microsoft-github-policy-service]

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