Allow search and multi-search to return raw data

[kevinwang-oversoul]

Change Summary

Overriden 2 function:

• Documents.search()
• MultiSearch.perform()

Why make this change

Currently the above 2 function will take a Codable type T and then try to decode it immediately after retrieving the data. This is not ideal because we lose the opportunities to implement a custom data parsing logic to it. E.g.

• Documents.search(): This function uses the default JSON decoder to decode the data. But sometimes we want to use other decoders, such as Firestore Decoder.
• MultiSearch.perform(): This function only takes 1 type of Codable, which is not ideal. E.g. for a social media app, when searching a post, we want to have some search suggestions. They are different types of data and the function won't be able to decode the multi search result.

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