Support SearchRequest compatibility for Msearch body and ScriptSource

[l-trotta]

Fixes #151.
Adds an overload to the setter for the body of Msearch and the template of ScriptSource, both of which require an instance of SearchRequestBody, by converting an instance of SearchRequest to SearchRequestBody.

Usage:

// Normal search request
SearchRequest searchRequest = SearchRequest.of(b -> b
    .size(10)
    .from(10)
    .query(q -> q
        .matchAll(m -> m)
    )
);

// Using it to build Msearch
MsearchRequest msearchRequestOverload = MsearchRequest.of(ms -> ms
    .searches(s -> s
        .header(h -> h.index("index"))
        .body(searchRequest)
    )
);

// Or script source
ScriptSource scriptSourceOverload = ScriptSource.of(s -> s
    .scriptTemplate(searchRequest)
);