Limit pipeline parallel execution per-branch

[lafriks]

Clear and concise description of the problem

Currently there is no way to reliability limit same pipeline with different commits not to be executed in parallel. This can be problematic if pipelines are used to do deployments and canceling running executing pipeline is not an option as that could break deployment.

Suggested solution

Allow to specify either in pipeline or in repo settings that pipelines for each branch are exclusive/should be queued (similarly how that it is possible to set cancel executing pipelines)

Additional context

In our use case if canceling helm deploy in k8s can result in broken deployment state

Validations

• Checked that the feature isn't part of the next version already [https://woodpecker-ci.org/faq#which-version-of-woodpecker-should-i-use]
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.