Add script to generate new TCLIService files

[gglanzani]

This PR add the generate.py function. When invoked, it will fetch the specified Thrift specification for HiveServer2, and generate the relevant Python files. If nothing is specified, it will fetch the spec from Hive 2.3.

Usage is

python generate.py <URL>

This PR should make it easier for developers to update the generated code more often.

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