[CLI] Allow config to be passed as a JSON o YAML string argument - not only as a file

[giorgioma]

I hope the title is self explanatory

Added the possibility to use

npx openapi-merge-cli --json-config '{
  "inputs": [
    ...
  ], 
  "output": ...
}
'

or

npx openapi-merge-cli --yaml-config '---
inputs:
  ...
output: ...
'

as added to docs

[giorgioma]

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