Use schema that doesn't come from a file

[ptodev]

It is sometimes useful to place the json schema in a file which may also contain other config. For example:

fields:
  not:

used:
  by:
  json_schema:

json_schema:

In the above example, we'd only need to give go-jsonschema the config which is under json_schema. The way I've been doing it so far is by making addFile a public function. Do you think this is ok?

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.77%. Comparing base (d963216) to head (97744ab).
Report is 65 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/generator/schema_generator.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #284      +/-   ##
==========================================
- Coverage   76.58%   75.77%   -0.81%     
==========================================
  Files          24       36      +12     
  Lines        1892     2089     +197     
==========================================
+ Hits         1449     1583     +134     
  Misses        354      354              
- Partials       89      152      +63     

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