Standardization for aggregation module

[0xjei]

This PR aims to implement a standard mechanism to reuse & aggregate in logic expressions multiple external contracts ("verifiers") using standard built-in Solidity language to optimize gas consumption and checks, leaving max freedom of implementation to the developer to advanced tricks (e.g., short circuit). These contracts have logic that is NOT controlled by the specific Checker or any Excubiae contract, but are managed externally by someone else. The Checker uses these contracts to verify specific verifiable attributes defined on the check(). These attributes verify evidence that is enforced by the `policy' that will use them. This diagram exemplifies what is the terminology relationship.

graph LR;
    Verifiers-->|verify validity of evidence for|Verifiable_Attributes;
    Verifiable_Attributes-->|defined and checked by|Checker;
    Checker-->|is enforced by|Policy;

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To avoid creating an over-engineered parser of logical expressions or new types of abstract Checker' contracts, we have abstracted at a high level taking it as inherent and necessary to any contract, implementing things directly in the core contracts. This way, any Base/AdvancedChecker' has a standard mechanism to handle multiple external contracts without having to code them manually (e.g., specifying a set of addresses and extending the current core), and the ability to specify a single encoded `evidence' for each one.

More details will be released in the documentation, which will be updated prior to release. This example [AdvancedVoting](https://github.com/privacy-scaling-explorations/excubiae/blob/main/packages/contracts/contracts/src/test/advanced/AdvancedVoting.sol) will be updated to demonstrate the new functionality.

TODO

• Basic Design & Implementation & Adaptation to current tests
•  Refactoring & Optimization
• update test x demonstration
• Naming
• 100% code coverage