Grant API key should mandate key name as create API key action

[ywangd]

The name of an API key is mandatory when creating an API key. It throws a 400 error if the name is missing. However, when using the grant API key action, it is possible to create a key without a name. This is because the name field is nested under api_key field for the input of grant API key action, i.e.:

{ 
    "...": "...",
    "api_key": {"name": "key-1"}
}

Although the name field is still mandatory, the parent api_key field is optional. Thus allow keys be created without names.

[elasticmachine]

Pinging @elastic/es-security (:Security/Authentication)