Implement placeholder search. Update tests

[eskombro]

About this PR:

As defined in meilisearch/meilisearch#771, MeiliSearch will implement placeholder search starting with v0.13.0

A search request containing no Query parameter (or a null value in that parameter) will send a response with a basic placeholder search showing ranked results, and will be as customizable as any other search request.

GO types and default initialization to zero value doesn't let us make a clear distinction between an uninitialized String (some nil value) and an empty String (some "" value) when all we are handling is a string. We need to be able to provide those two options in order to implement placeholder search, which expects no q parameter (no query) or a null value.

An option to do this would be using a string pointer that can be uninitialized (nil), contain an empty string, or contain a valid string. The problem with this is that it would add some unnecessary complexity to the whole search experience, as the user would need to always use pointers towards a string to perform the search. So adding a new functionnality results in making search experience work.

The most practical solution to this from my POV is to add a boolean value to the SearchRequest struct where the user indicates if he wants to perform a placeholder search. This won't be breaking, completey optional, and really simple to use.It can work as in:

resp, err = client.Search(indexUID).Search(SearchRequest{
	PlaceholderSearch: true,
})

Which would return the placeholder search results, and would be clearly different than

resp, err = client.Search(indexUID).Search(SearchRequest{
	Query: "",
})

if PlaceholderSearch is set to true, Query will be totally ignored for that request

Which would return no results

⚠️ Tests will not pass until v0.13.0 of MeiliSearch is released

Closes #69

[qdequele]

How about make like Algolia and write multiple functions for that?

resp, err = client.Search(indexUID).Search(SearchRequest{
    Query: "",
    ...SearchParameters
})

resp, err = client.Search(indexUID).PlaceholderSearch(PlaceholderSearchRequest{
    ...PlaceholderSearchParameters
})

[eskombro]

Thanks @qdequele!

I agree with you and it seems like it would be the 'Go' way to handle this kind of situation. Talking with the integration team we also think that as a user it is easier to just play with one parameter in the request parameters object to be able to switch from 'normal search' to a 'placeholder search', and it might be annoying to be forced to use a different method with a different parameters object. So maybe we can leave for now the boolean variable to activate placeholder.