Refactor pipeline-generator to use CommandLineParser

[benbp]

I'm extending pipeline-generator to enable scenarios detailed in #2572. To do this, pipeline-generator will have multiple sub-commands:

pipeline-generator generate   # Current functionality, generate/sync pipelines from yaml files
pipeline-generator bootstrap  # (new) Create or update service principals and service connections for pipeline usage
pipeline-generator onboard    # (new) Set up keyvaults and/or access policies for new teams onboarding to Bring Your Own Subscription

As part of adding sub-command support, this PR updates the project to use CommandLineParser in line with our other C# CLI projects. I have also set up defaults for a lot of the common values to make local invocation much more concise, for example:

pipeline-generator generate --path /home/ben/sdk/azure-sdk-for-go/sdk -x go -d "\go" -r Azure/azure-sdk-for-go -c up -v 64 --set-managed-variables

After merge and tool release, I will put up a separate PR that includes the yaml config changes to call the tool correctly alongside the version bump.

[weshaggard]

Looks reasonable to me

[benbp]

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[github-actions]

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