fix: fetch json_library option on compile-time

[zoedsoupe]

Problem

We've introduced the json_library config option but only applied it to the main library (this one)

Trying to override on the other ones, it got shown that we need a compile-time value since other libraries - like storage-ex derives the Encoder protocol for structs, so a runtime value can't be used

Solution

Transform the option on compile-time, so users need to set it on config.exs exactly like the phoenix library does (although it may fetch fom runtime too)

Rationale

[zoedsoupe]

with that one i can ship the usage of this option across the rest of libraries

[zoedsoupe]

i also tried to respect the standardized api for Encoder protocol across the 3 main encoding libraries: Jason, Poison and the new 1.18 native JSON module

[Ziinc]

I think we can keep it compile time for now after some thought, quite a minor thing.
Might be worthwhile to add to docs to point out that it is compile time only.