Move building of pipeline metadata to its own step in chain

What does this MR do and why?

Describe in detail what your merge request does and why.

As suggested in !101399 (comment 1147850925), create a Chain::PopulateMetadata class and add it to the chain of steps in CreatePipelineService. Move the building of the Ci::PipelineMetadata model from the Chain::Populate class to the Chain::PopulateMetadata class.

This MR also makes sure that the chain is broken (pipeline creation errors out) if there is an error in the Ci::PipelineMetadata model.

#372538 (closed)

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