[Workflows] Explore Pipeline Concurrency Support in Workflows

[dibyom]

Feature request

Provide a way to configure how many runs for a given can be running at once. This feature has come up a few times across a bunch of issues in Pipelines (and was added to the roadmap):

• Idea: Pipeline Mutexes pipeline#2828
• Controlling max parallel jobs per pipeline pipeline#2591
• provide a pipeline concurrency limit pipeline#1305
• Add concurrency limit to roadmap pipeline#3130

In addition, this was also proposed as a experimental project: #699

Use case

• See the linked issues

Next step: Gather requirements/use cases.

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