[ML] Parse an array as root element in custom service response parser

[fdartayre]

Description

The custom service response parser expects an object (map) as the root element of the JSON response from the custom service.

This enhancement would permit to parse the following response using this json_parser string $[*].embeddings[*]:

[
    {
        "embeddings": [
            0.0,
            0.0,
            0.0,
            0.0,
            0.0,
            0.0,
            ...
            0.0,
            0.0,
            0.0,
            0.0,
            0.0,
            0.0,
            0.0,
            0.0,
            0.0
        ]
    }
]

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)