feat: Add NexusViewer

[rackstar]

Context

Closes #1166

Add NexusViewer to provide a unified view of system-wide data from various contracts within the Nexus Mutual platform

NOTE: this PR is behind https://github.com/NexusMutual/smart-contracts/pull/1164

Changes proposed in this pull request

• Add NexusViewer
• Add mocks
• Add fork tests
• Add unit tests

Test plan

• Add fork tests
• Add unit tests

Checklist

• [x] Rebased the base branch
• [x] Attached corresponding Github issue
• [x] Prefixed the name with the type of change (i.e. feat, chore, test)
• [x] Performed a self-review of my own code
• [x] Followed the style guidelines of this project
• [x] Made corresponding changes to the documentation
• [x] Didn't generate new warnings
• [x] Didn't generate failures on existing tests
• [x] Added tests that prove my fix is effective or that my feature works

Review

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• :cake: = Nice to have but not essential.
• :bulb: = Suggestion or a comment based on personal opinion
• :hammer: = I believe this should be changed.
• :thinking: = I don’t understand something, do you mind giving me more context?
• :rocket: = Feedback

[rackstar]

ready for review. but marking as DRAFT as it should not be merged until https://github.com/NexusMutual/smart-contracts/pull/1170 and https://github.com/NexusMutual/smart-contracts/pull/1164 are merged first