Create feature flag for the support of content parts

[adubovik]

OpenAI messages may have content represented as a list of sub-messages called content parts.

Whether deployment supports requests with content parts or not should be revealed in the Core listing.
E.g. DIAL Chat may turn on a hypothetical multi-part mode if a deployment supports content parts.

The feature should be called contentPartsSupported:

{
    "features": {
        "contentPartsSupported": true
    }
}

The issue is related to epam/ai-dial-sdk#163 and #506.