fix: correctly handle different json libraries apis/decoding opts

[zoedsoupe]

Problem

introduced on #74, users could customize the json library used to decode/encode data across the supabase-ex libraries, although functional, we've introduced 2 minor bugs:

1. the otp app defined to retireve the json_library opt was mismatching the library ones (supabase_potion)
2. with the new "feature", we lost the ability to convert the keys to atoms or not, which other supabase-ex libraries relies on

Solution

1. correctly handles the otp app
2. try to use the standard API for decode/1/encode_to_io_data!/1 since all json libraries have at least those functions with same interface and convert the keys internally

Rationale

N/A

[zoedsoupe]

will be used internally on the other libraries to avoid duplicating this logic across the ecosystem