refactor: default to OTP json, drop thoas dependency

[Taure]

Summary

• Switches the default json_lib from thoas to the OTP json module and removes thoas from the dependency list.
• json_lib remains configurable so users can plug in any module that follows the OTP contract: encode/1 -> binary() | iodata(), decode/1 -> Term (raises on bad input).
• nova_request_plugin wraps decode/1 in try/catch to keep returning 400 on malformed JSON.
• nova_jsonlogger:encode/2 wraps its output in iolist_to_binary/1 so the formatter still returns a binary regardless of backend.
• Inline jsonlogger tests and the configuration guide updated to the OTP json contract.

Test plan

• rebar3 compile
• rebar3 xref
• rebar3 dialyzer
• rebar3 eunit (8/0 — same as master baseline)

[burbas]

We need to have a discussion when to use try..catch for these kinds of operations and when not to.

[burbas]

I add my comment to this block since it showcase my point;

Here we do try...catch but in the rest of the code we just let it crash. That we can not have - we need to discuss a strategy on how to handle these kinds of problems. Do we crash or do we catch it? It might be that we want to do different things in different situations - but think we need to discuss this more broadly.