Integrate automated toolings for testing smart contracts

[matjazv]

Description

Introduce automated tooling to:

• Discover coverage gaps in the testing of smart contracts
• Look for common issues in the smart contracts and the backend components

Tools which may be used:

• Slither
• slither-mutate
• Echidna

Acceptance Criteria

• GitHub actions are using (some) of above tools to additionally verify smart contracts correctness and common issues
• It's possible to execute (some) of above tools locally on a PC to check their status before pushing code to GH

Additional Information

slither-mutate is probably not a good candidate to use it for GH actions because it was reported by auditors than this tool needed around 4 hours for its execution to be completed. It may be still used for local testing.