Calling `json_list_template& operator=(const C& list)` results in oversized allocation

[robertbindar]

Calling json_list_template& operator=(const C& list) results in oversized allocation if the list you want to assign from is big enough.
Given there's no option of a move assignment operator, we should either implement one or optimize the copy operator to avoid the large allocation.

Ref scylladb/scylladb#26736