Added service and mutation to set user namespace commit email

What does this MR do and why?

Follow up of !107525 (merged) where NamespaceCommitEmailType was exposed 🙂

This MR aims at providing the GraphQL mutation to set the users' namespace commit email 🙂

Screenshots or screen recordings

N.A.

How to set up and validate locally

Use a mutation like this one to change the user commit email based on the namespace:

mutation {
  userSetNamespaceCommitEmail(input: {
    userId: "gid://gitlab/User/20",
    namespaceId: "gid://gitlab/Namespace/29",
    emailId: "gid://gitlab/Email/7"
  }) {
    namespaceCommitEmail {
      id
    }
    errors
  }
}

Passing null or omitting email_id will remove an existing user namespace commit email.

You will need to check the records are successfully create via GraphQL, the rails console or in the database directly:

% rails c
=> Users::NamespaceCommitEmail.last

% gdk psql
=# select * from namespace_commit_emails;

query getUser {
  user(username: "ROOT") {
    namespaceCommitEmails { 
      nodes {
        id
        namespace {
          fullPath
        }
        email {
          email
        }
      }
    }
  }
}

MR acceptance checklist

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