fix(api)!: only accept json content

[noaccOS]

1. Clients can not send urlencoded/multipart requests.
   BREAKING CHANGE: this means that eg an appengine request on the endpoint
   interface_values_path POST /v1/:realm_name/devices/:device_id/interfaces/:interface/*path_token
   which previously could be successful with an urlencoded body of data=2 will not be successful anymore.

2. Remove pass: ["*/*"]
   From the Plug.Parsers documentation, "This plug will raise Plug.Parsers.UnsupportedMediaTypeError by default if the request cannot be parsed by any of the given types and the MIME type has not been explicitly accepted with the :pass option".
   I believe this to be the desired behavior, as the application should return error code 415 if it is not parsable by one of the specified parsers.

Closes #263

[codecov]

Codecov Report

Merging #831 (16c7f32) into master (e0fd426) will decrease coverage by 0.01%.
Report is 12 commits behind head on master.
The diff coverage is 33.33%.

❗ Current head 16c7f32 differs from pull request most recent head f3f4489. Consider uploading reports for the commit f3f4489 to get more accurate results

@@            Coverage Diff             @@
##           master     #831      +/-   ##
==========================================
- Coverage   67.42%   67.42%   -0.01%     
==========================================
  Files         264      264              
  Lines        6429     6428       -1     
==========================================
- Hits         4335     4334       -1     
  Misses       2094     2094              

Files Changed	Coverage Δ
...ne_api/lib/astarte_appengine_api/device/queries.ex	80.92% <0.00%> (ø)
...ib/astarte_data_updater_plant/data_updater/impl.ex	62.16% <0.00%> (ø)
..._management/lib/astarte_realm_management/engine.ex	78.46% <100.00%> (ø)

... and 8 files with indirect coverage changes