Fluent API for composite aggregation sources

[stijnherreman]

Is your feature request related to a problem? Please describe.
There doesn't seem to be a fluent API to define composite aggregation sources.

Describe the solution you'd like
I'd like an API similar to .MultiTerms(...).

var response = await client.SearchAsync<Foo>(search => search
    .Aggregations(aggregations => aggregations
        .Add("foo", aggregation => aggregation
            .MultiTerms(multiTerms => multiTerms
                .Terms(
                    terms => terms
                        .Field(foo => foo.Bar),
                    terms => terms
                        .Field(foo => foo.Baz)
                )
            )
        )
    )
    // ...

Describe alternatives you've considered
Currently using the non-fluent API. I was a bit confused initially by the method taking a ICollection<IDictionary<string, CompositeAggregationSource>> parameter.

var response = await client.SearchAsync<Foo>(search => search
    .Aggregations(aggregations => aggregations
        .Add("foo", aggregation => aggregation
            .Composite(composite => composite
                .Sources(new[]
                {
                    new Dictionary<string, CompositeAggregationSource>()
                    {
                        { "bar", new() { Terms = new() { Field = "bar" } } }
                    },
                    new Dictionary<string, CompositeAggregationSource>()
                    {
                        { "baz", new() { Terms = new() { Field = "baz" } } }
                    }
                })
            )
        )
    )
    // ...

Additional context
I'm using a composite aggregation because I need to stream all buckets.
The example at #7822 (comment) was quite helpful.