Add CreateRunnerService

What does this MR do and why?

As part of building out the mutation to create runners, this MR is introducing a new service to create runners. The policies to authorize creating runners with this method are behind the feature flag create_runner_workflow and are introduced in !109531 (merged). The feature flag has already been created (MR).

registration_type: :authentication_token is added to the params for the Ci::Runner constructor to facilitate new token creation. This was added in !111946 (merged).

Screenshots or screen recordings

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How to set up and validate locally

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MR acceptance checklist

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