Added GraphQL mutation to set time estimate on issuables

What does this MR do and why?

This MR changes the GraphQL mutations to update issues and merge requests to also allow users to set time estimate 🙂

Screenshots or screen recordings

N.A.

How to set up and validate locally

Preface:

• time estimate is expressed using "human readable" format, just like on the /estimate quick action
• passing a zero-based time estimate (eg 0h) will reset the time estimate (to 0)

Find a merge request on a project and use this mutation to change its time estimate:

mutation {
  mergeRequestUpdate(input: {
    projectPath: "<project full path>",
    iid: "<merge request id>",
    timeEstimate: "<time estimate>"
  }) {
    mergeRequest {
      timeEstimate
    }
  }
}

Find an issue on a project and use this mutation to change its time estimate:

mutation {
  updateIssue(input: {
    projectPath: "<project full path>",
    iid: "<issue id>",
    timeEstimate: "<time estimate>"
  }) {
    issue {
      timeEstimate
    }
  }
}

Make sure that:

• the time estimate was changed
• a system note was created

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.