Implement Standard Slashing Protection JSON With Importing Logic

[shayzluf]

Part of #7813

What type of PR is this?

Feature

We have written up a design document for this feature here.

What does this PR do? Why is it needed?

This PR implements the standard slashing interchange format defined by https://eips.ethereum.org/EIPS/eip-3076. It allows for unmarshaling an input JSON file and converting it into our internal representation of attester and proposal slashing protection. Then, we can import this representation into our database atomically.

Among the features included are:

• Reading a JSON file and ensuring its necessary fields are not junk data
• Checking against the version field of the parsed JSON
• Allowing for duplicate public keys in the slashing protection data with possibly different signed blocks and attestations
• Comprehensive tests for ensuring we handle duplicate data, bad data, and good data
• Tests for different helper functions, especially string parsers
• Validate genesis root [Tracking] - Standard Slashing Protection Interchange Functionality #7813
• Validate whether the data we are importing is slashable with respect to our database [Tracking] - Standard Slashing Protection Interchange Functionality #7813
• Validate whether the data we are importing is slashable with respect to other entries in the JSON file [Tracking] - Standard Slashing Protection Interchange Functionality #7813

Other notes for review

The underlying representation of proposal protection assumes that everything imported from the JSON will have a signing root, however, signing roots are optional in the EIP. The tracking issue mentions this as an open action item to tackle in a future PR.