feat: allow overriding --language_out parameter to protoc

[alexander-fenster]

This feature allows to override the default value for --language_out (--php_out, --python_out, etc.) parameter of protoc invocation with an arbitrary string.

E.g. if you want to make PHP protoc invocation have --php_out=key=value:/path/to/output-dir, add this to Artman YAML config:

artifacts:
- name: php_gapic
  language_out_override: "key=value:{root}"

The {root} placeholder will be replaced with the actual output directory. The same logic will work for any other language as well.

Cc: @pierrickvoulet