Implement get_open_id_token

Introduce the CognitoIdentity's GetOpenIDToken endpoint to retrieve a
JWT tuple from Cognito's Identity Pool for a given IdentityId.

IMPLEMENTATION_COVERAGE.md

@@ -815,16 +815,16 @@
 - [ ] update_user_profile
 
 ## cognito-identity - 0% implemented
-- [ ] create_identity_pool
+- [X] create_identity_pool
 - [ ] delete_identities
 - [ ] delete_identity_pool
 - [ ] describe_identity
 - [ ] describe_identity_pool
-- [ ] get_credentials_for_identity
+- [X] get_credentials_for_identity
-- [ ] get_id
+- [X] get_id
 - [ ] get_identity_pool_roles
-- [ ] get_open_id_token
+- [X] get_open_id_token
-- [ ] get_open_id_token_for_developer_identity
+- [X] get_open_id_token_for_developer_identity
 - [ ] list_identities
 - [ ] list_identity_pools
 - [ ] lookup_developer_identity

moto/cognitoidentity/models.py

@@ -95,6 +95,15 @@ class CognitoIdentityBackend(BaseBackend):
             })
         return response
 
+    def get_open_id_token(self, identity_id):
+        response = json.dumps(
+            {
+                "IdentityId": identity_id,
+                "Token": get_random_identity_id(self.region)
+            }
+        )
+        return response
+
 
 cognitoidentity_backends = {}
 for region in boto.cognito.identity.regions():

moto/cognitoidentity/responses.py

@@ -35,3 +35,8 @@ class CognitoIdentityResponse(BaseResponse):
         return cognitoidentity_backends[self.region].get_open_id_token_for_developer_identity(
             self._get_param('IdentityId') or get_random_identity_id(self.region)
         )
+
+    def get_open_id_token(self):
+        return cognitoidentity_backends[self.region].get_open_id_token(
+            self._get_param("IdentityId") or get_random_identity_id(self.region)
+        )

tests/test_cognitoidentity/test_cognitoidentity.py

@@ -68,7 +68,7 @@ def test_get_open_id_token_for_developer_identity():
         },
         TokenDuration=123
     )
-    assert len(result['Token'])
+    assert len(result['Token']) > 0
     assert result['IdentityId'] == '12345'
 
 @mock_cognitoidentity
@@ -83,3 +83,15 @@ def test_get_open_id_token_for_developer_identity_when_no_explicit_identity_id()
     )
     assert len(result['Token']) > 0
     assert len(result['IdentityId']) > 0
+
+@mock_cognitoidentity
+def test_get_open_id_token():
+    conn = boto3.client('cognito-identity', 'us-west-2')
+    result = conn.get_open_id_token(
+        IdentityId='12345',
+        Logins={
+            'someurl': '12345'
+        }
+    )
+    assert len(result['Token']) > 0
+    assert result['IdentityId'] == '12345'