Add a service to create a merge request from a security finding

What does this MR do and why?

Describe in detail what your merge request does and why.

As part of issue #368311 (closed), we are updating the Projects::VulnerabilityFeedbackController to create Vulnerability on the fly for a given Finding.

When interacting with security findings, the user has the option to create a MergeRequest for the given Finding. This Merge Request adds a new service to create a MergeRequest for a given finding.

This new Service will also create a new Vulnerability if needed and a Merge Request Link wrapped in a transaction as discussed here.

• This will not create a new Vulnerabilities::Feedback object but only a Vulnerability.
• We will need to set the present_on_default_branch to false for these new vulnerabilities

This new Service will be used by the Projects::VulnerabilityFeedbackController.

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MR acceptance checklist

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