Add pipelineSchedulePlay mutation

Continued from community contributors MR. Description is from there.

What does this MR do and why?

Added pipelineSchedulePlay mutation.

Mutation Example

mutation PipelineSchedulePlay($input: PipelineSchedulePlayInput!) {
  pipelineSchedulePlay(input: $input) {
    errors # [String]: Array of error messages. Empty if the mutation is successful.
    pipelineSchedule { id, nextRunAt}
  }
}

input PipelineSchedulePlayInput {
  clientMutationId: String
  # ID of the pipeline schedule to mutate.
  id: CiPipelineScheduleID!
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #372129 (closed)