[DOCS] Api key format doesn't match current UI

[Dig-Doug]

The ApiKey credential currently asks for a key id and an api key. However, in the elasticsearch cloud UI, API keys don't provide these components separately. Instead, they provide base64(keyid + ":" + key) as one string. Since you also give API keys a name, my initial thought was to provide the api key name as the id and then the base64 key as the value.

Either the rust client should be updated to take this format or the docs should be updated to make it clear how to get these values.

[swallez]

In PR #238 we added a new EncodedApi credential that takes a single string so that both representations of API keys can be used.