Do not encode json request body

[zoedsoupe]

Problem

The problem is that the lib was encoding the request body json twice, so it was being sent as a string and PostgREST was decoding as a raw string instead of a json itself.

Solution

Stops to encode the request body at all and let the Finch http client decide to do it once, also it handle correctly media types headers accept

Rationale

[zoedsoupe]

this closes #2 and also updates elixir to v1.18 (with improvements in the type system) and also init a supabase local project