[Checkpointing] ExitEvent storage and proof generation

[goran-ethernal]

Description

Added functions to store and query exit events in state.go.
Added event root hash calculation which will be used as a parameter in block extra (one of the parameters for checkpoint), that gets signed by validators.
Added exit proof generation for users that want to exit.
Added relevant UTs for positive and negative cases.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (feature/v3-parity@c9b71a1). Click here to learn what that means.
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❗ Current head a6cbda3 differs from pull request most recent head 213aa83. Consider uploading reports for the commit 213aa83 to get more accurate results

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[Stefan-Ethernal]

Generally looks good, but found some minor points to be addressed.