`mock_get` was created only for forward_service use, thus it only checks for
aggregate functions if no declared function was found.
The reason for this function is, there is no serialization of `cql3::selection::selection`,
so functions lying underneath these selections has to be refound.
Most of this code is copied from `functions::get()`, however `functions::get()` is not used because it requires to
mock or serialize expressions and `functions::find()` is not enough,
because it does not search for dynamic aggregate functions
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