Adds pathItem common parameters

[spacether]

• Adds pathItem common parameters which are parameters defined at the path Item level(not at operation level)
• py files written for pathItem parameters
• parameters are used in endpoint operations if needed. They can be overridden. That override use case is included.
• refactors CodegenOperation to use a new ParameterCollection class to hold all/path/query/header/cookie parameters. Uses that class to store operation parameters and pathItem parameters for an operation
• added 3 tests of python routes that use path Item common parameters, one where the parameter is overridden

PR checklist

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  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

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