feat(ci/get): add with-downstream-pipelines flag

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Description

This change adds the flag for glab ci get called --with-downstream-pipelines that also fetches downstream pipelines if there are any.

I have added the bridge jobs to the struct that was already used for the merged response

type PipelineBridge struct {
	Bridge    *gitlab.Bridge             `json:"bridge"`
	Pipeline  *gitlab.Pipeline           `json:"pipeline"`
	Jobs      []*gitlab.Job              `json:"jobs"`
	Variables []*gitlab.PipelineVariable `json:"variables"`
}

type PipelineMergedResponse struct {
	*gitlab.Pipeline
	Jobs      []*gitlab.Job              `json:"jobs"`
	Bridges   []PipelineBridge           `json:"bridges"`
	Variables []*gitlab.PipelineVariable `json:"variables"`
}

Related Issues

Resolves [#7933]

How has this been tested?

I have extended the testing that was already there for glab ci get, to include tests with the flag --with-downstream-pipelines.

I have also built the binary and run it against our GitLab self-hosted instance, which worked.

Screenshots (if appropriate):

Screenshot running in a real scenario.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap