A minimum and maximum for hasMany in relationship

[Chrinsieboy]

When you have a relationship to staffmembers and you want a minimum then you cant do that at the moment.
Maybe add a feature so you can like this:

{
  slug: 'staff-collection',
  fields: [
    {
      name: 'staff',
      type: 'relationship',
      relationTo: 'members',
      hasMany: true,
      admin: {
          // option 1
          minItems: 1,
          maxItems: 3,

          // option 2
          min: 1,
          max: 3
      }
    }
  ]
}

[DanRibbens]

I needed this so I made it a reality: #2237

Since validation happens in both the admin UI and the backend I opted to putmin and max at the top instead of within admin.