[BE] Config: New graphql mutation to create fine-grained PAT

We need a mutation that allows a fine-grained personal access token to be created.

Proposed mutation

mutation {
  personalAccessTokenCreate(input: {
    name: "Token name"
    description: "Token description"
    expiresAt: "2025-10-03T02:10:43Z"
    permissions: [
      { value: "SOME_PERMISSION", namespace: "gid://gitlab/Group/1" }, # group permission
      { value: "SOME_PERMISSION", namespace: "gid://gitlab/Project/2" }, # project permission
      { value: "SOME_PERMISSION", namespace: "gid://gitlab/User/3" }, # user permission
      { value: "SOME_PERMISSION", namespace: null } # instance permission
    ]
  }) {
    errors
    personalAccessToken {
      ...
    }
  }
}

Field	Description
name	Name of the token
description	Description of the token
expiresAt	Timestamp that the token expires at
permissions	Array of permissions that the token grants. The value is the token's unique identifier. namespace is the namespace of the boundary that the token belongs to. It can either be a group, user, project, or null (meaning an instance permission)