feat: introduce login privilege instead of session and usage

Summary

• removes usage from picodata privilege types
• renames session to login (as that's how we are going to name it in docs, let's have it in code also)
• decouples picodata privilege types from tarantool ones

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• Relates to #443 (closed)
• Target branch: master
• Docs follow-up: planned as part of #443 (closed)