Improve detection of base64 execution via codecs and importlib dynamic imports

[dwBruijn]

Hello,

Added patterns to exec-base64 rule to detect base64 decoding using codecs.decode(), importlib.import_module("base64").b64decode(...) and importlib.import_module("codecs").decode(...) (fairly common when trying to avoid detection).

These patterns were not being detected:

codecs.decode(..., 'base64')
importlib.import_module('base64').b64decode(...)
importlib.import_module('codecs').decode(..., 'base64')

[sobregosodd]

Thanks for contributing @dwBruijn !