feat: Add Python goodies

[tmathern]

• Make Stream class usable with context manager (with keyword)
• When loading the bound native lib, check needed functions are here to avoid bad surprises
• Streamline error handling
• Rewrite one test file to use unittest instead of pytest (pytest command still can run the tests) to have only one framework used
• Found a bug where c2pa_builder_add_ingredient instead of c2pa_builder_add_ingredient_from_stream was called, fixed it (The symbol is not in the current lib)
• Make resources cleaning for Reader, Builder, Signer and Stream more robust (esp. avoid double free)
• Probably some other minor things I forgot while working on the above!

Also tested these changes with other implementations we're working on and tests passed there too

[gpeacock]

Thanks for this!