Fix FieldRule (#2362)

* Fix FieldRule

* `username` can also be a list,
* `realm.name` is the field name, using an intermediate `realm` object does not work.
* `metadata` is not a key: keys are of the form `metadata.key = value`

* Apply suggestions from code review

Co-authored-by: Laurent Saint-Félix <laurent.saintfelix@elastic.co>

* Run `make contrib`

* Update RoleMappingRule.ts

Co-authored-by: Laurent Saint-Félix <laurent.saintfelix@elastic.co>

* Address review comments

---------

Co-authored-by: Laurent Saint-Félix <laurent.saintfelix@elastic.co>

Author: pquentin

output/schema/schema.json

@@ -26,21 +26,19 @@ import { Name, Names } from '@_types/common'
 export class RoleMappingRule {
   any?: RoleMappingRule[]
   all?: RoleMappingRule[]
+  // `field` should have been defined as SingleKeyDictionary<String, ScalarValue|ScalarValue[]>
+  // However, this was initially defined as a container with a limited number of variants,
+  // and was later made non_exhaustive to limit breaking changes.
   field?: FieldRule
   except?: RoleMappingRule
 }
 
 /**
  * @variants container
+ * @non_exhaustive
  */
 export class FieldRule {
-  username?: Name
+  username?: Names
   dn?: Names
   groups?: Names
-  metadata?: UserDefinedValue
-  realm?: Realm
-}
-
-export class Realm {
-  name: Name
 }