Add dataset to `SearchInvokeValue`

[kuojianlu]

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Is your feature request related to a problem? Please describe.
For Adaptive Card dynamic search (example), dataset will be included in Activity.Value. For example,

But dataset is not included in SearchInvokeValue. Is there any reason behind this design?

Describe the solution you'd like
Add dataset to SearchInvokeValue. For example:

namespace Microsoft.Bot.Schema
{
    public class SearchInvokeValue
    {
        [JsonProperty("kind")]
        public string Kind { get; set; }

        [JsonProperty("queryText")]
        public string QueryText { get; set; }

        [JsonProperty("queryOptions")]
        public SearchInvokeOptions QueryOptions { get; set; }

        [JsonProperty("dataset")]
        public string Dataset { get; set; }

        [JsonProperty("context")]
        public object Context { get; set; }
    }
}

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.