Add parameter to configure mirror_trigger_builds

What does this MR do and why?

When someone uses the GitLab UI to configure a mirror, one of the configuration options they are presented with is "Trigger pipelines for mirror updates". When creating a pull mirror using the API, that option is not mentioned, and knowing you might want it and then finding the available option is non-trivial. It is helpful to enumerate it as a possible option in this short list.

This update was suggested by a customer who experienced this frustration in this internal ZD ticket

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