Drop the use of github.com/mailru/easyjson #5361
Description
As mentioned here, considering easyjson is used for non-performance-critical bits, and even more now with the introduction of encoding/json/v2, it no longer makes sense to keep using it.
Therefore, the agreement is to get rid of it, but we can't do it before k6 v2, as it's currently used in some public API types, which is part of our versioning and stability guarantees.