Add basic EASExcubia implementation

[0xjei]

Description

This PR introduces the basic form of an EASExcubia extension. This excubia allows the gate to check if the passerby has a given attestation with a given schema in order to pass. This contract implements a mapping to avoid using the same attestation to pass twice.

Other information

Please, note that this is a porting of the MACI EAS Gatekeeper.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have run yarn format and yarn compile without getting any errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[ctrlc03]

Good stuff thank you!