Make it possible to limit the maximum concurrent deployments that can be performed by a single PipeCD agent

[nghialv]

What would you like to be added:

Why is this needed:

Currently, we are using the PipeCD agent to deploy a large number of Terraform applications. However, the agent starts running all deployments that have been triggered without considering its processing ability. As a result, our PipeCD agent often gets stuck when multiple deployments are triggered simultaneously.

Therefore, I wish we had a way to specify a limit number in the PipeCD agent configuration to prevent it from burning out.

Some other considerations to take into account:

The limit number should be applied to each type of application (e.g., Terraform apps may require more resources to handle than other types of apps). Deployments triggered by plan-preview should also be counted in the limit.

[moko-poi]

👍

[github-actions[bot]]

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