Add spaceType as a top level parameter while creating vector field.

Description

Add spaceType as a top level parameter while creating vector field. With this change a user can change the spaceType for the vector field without defining the method parameter in the mapping. With this change Disk based vector search OOB experience is improved.

Things to be added:

Tests will be added in followup PRs

How testing is done:

I have performed manual testing by running following create index request.

Success

{
    "settings": {
        "index": {
            "number_of_shards": 1,
            "number_of_replicas": 0,
            "refresh_interval": "1s",
            "knn": true
        }
    },
    "mappings": {
        "properties": {
            "my_vector2": {
                "type": "knn_vector",
                "dimension": 4,
                "mode": "on_disk",
                "compression_level": "32x",
                "space_type": "innerproduct"
            }
        }
    }
}

Success and hits the legacy method and use the innerproduct space type

{
    "settings": {
        "index": {
            "number_of_shards": 1,
            "number_of_replicas": 0,
            "refresh_interval": "1s",
            "knn": true
        }
    },
    "mappings": {
        "properties": {
            "my_vector2": {
                "type": "knn_vector",
                "dimension": 4,
                "space_type": "innerproduct"
            }
        }
    }
}

Check List

[X] New functionality includes testing.
[X] New functionality has been documented.
[X] API changes companion pull request created.
[X] Commits are signed per the DCO using --signoff.

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opensearch-project / k-NN