Resolve "Caching of DOI based tools does not work"

Closes #152 (closed)

Summary

• added json caching in language.actorconfig and refactoring
• refactored downloaded_if_needed(), supporting zenodo checksums
• adapted test cases in test_actor_config to refactoring in dowload_if_needed

Iteration

• merged SessionProvider
• refactored duplicate code in actorconfig
• optimized checksum / etag caching
• precaches zenodo checksum per cache update to save additional request