feat: add abstract vRNG consumer

[coffeexcoin]

PR-Codex overview

This PR introduces a new vrng module for handling verified random number generation in smart contracts. It includes interfaces, error handling, and consumer contracts to facilitate random number requests and callbacks.

Detailed summary

• Added IVRNGSystem and IVRNGSystemCallback interfaces.
• Created VRNGConsumer and VRNGConsumerAdvanced contracts for randomness requests.
• Implemented error handling in Errors.sol.
• Developed mock implementations for testing: MockVRNGSystem, MockVRNGConsumerImplementation, and MockVRNGConsumerAdvancedImplementation.
• Updated README.md to include vrng module details.
• Added tests in VRNGConsumerTest.sol for various scenarios and validations.

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