Add pipelineScheduleCreate mutation

What does this MR do and why?

• Added pipelineScheduleCreate mutation
• Added PipelineSchedulVariableInput Type
• Added PipelineSchedule GraphQL Type
• Added PipelineScheduleVariable GraphQL Type

Mutation Example

mutation PipelineScheduleCreate($input: PipelineScheduleCreateInput!) {
  pipelineScheduleCreate(input: $input) {
    errors
    pipelineSchedule {
      active
      cron
      cronTimezone
      description
      id
      owner {
        publicEmail
      }
      project {
        path
      }
      ref
      variables {
        nodes {
          id
          key
          raw
        }
      }
    }
  }
}

input PipelineScheduleCreateInput {
  """
  Indicates if the pipeline schedule is active or not.
  """
  active: Boolean

  """
  A unique identifier for the client performing the mutation.
  """
  clientMutationId: String

  """
  Cron expression of the pipeline schedule.
  """
  cron: String!

  """
  Cron timezone of the pipeline schedule.
  """
  cronTimezone: String!

  """
  Description of the pipeline schedule.
  """
  description: String!

  """
  Full path of the project the pipeline schedule is associated with.
  """
  projectPath: ID!

  """
  Ref of the pipeline schedule.
  """
  ref: String!

  """
  Variables for the pipeline schedule.
  """
  variables: [PipelineScheduleVariableInput!]
}

input PipelineScheduleVariableInput {
  """
  Name of the variable.
  """
  key: String!

  """
  Value of the variable.
  """
  value: String!

  """
  .
  """
  variableType: CiVariableType
}

enum CiVariableType {
  ENV_VAR
  FILE
}

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)