GetRoleMappings deserialisation exception if response includes array of dn strings

[stevejgordon]

NEST/Elasticsearch.Net version: 7.10.1
Elasticsearch version: 7.10

Description of the problem including expected versus actual behavior:
When a role mapping contains an array of possible distinguished names, it causes a deserialisation error in NEST.

Steps to reproduce:

1. Add the following role mapping to the Elasticsearch cluster by issuing:

PUT /_security/role_mapping/test%20admin%20role%20mapping
{
    "enabled" : true,
    "roles" : [
      "apm_user"
    ],
    "rules" : {
      "any" : [
        {
          "field" : {
            "dn" : [
	      "CN=Bloggs Joe abcdef01,OU=Users,OU=_Central,OU=S1000,OU=SG001,DC=ad001,DC=example,DC=net",
              "cn=bloggs joe abcdef02,ou=usersfunctional,ou=_central,ou=accadm,OU=SG001,DC=ad001,DC=example,DC=net"
            ]
          }
        }
      ]
    },
    "metadata" : { }
}

2. Use NEST to get the role mappings

var client = new ElasticClient();
var resp = client.Security.GetRoleMapping();

Expected behavior
The response should be successfully deserialised.

** Actual behavior**
An UnexpectedElasticsearchClientException is throw in the deserialisation code.

[stevejgordon]

Fixed in #5271