feat: implement JCS-compliant SHA-256 hashing for Attest message segm…

[bulltickr]

…ents and define related segment types.

Description

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update (changes to documentation only)
• Refactoring (no functional changes, no API changes)
• Tests (adding or updating tests)
• Chore (maintenance tasks, build tools, CI/CD)

Related Issues

Fixes # (issue number)
Related to # (issue or PR number)

Testing Performed

Manual Testing

• Tested locally with go build and ./attest
• Tested specific command:

  attest <command> --flags

Automated Tests

• All existing tests pass

  go test ./...

• New tests added

  go test -v ./pkg/... -run TestNewFeature

• Tests with race detector

  go test -race ./...

• Coverage maintained or improved

  go test -coverprofile=coverage.txt ./...

SDK Testing

• Python SDK tested

  cd sdk/python && pytest

• JavaScript SDK tested

  cd sdk/js && npm test

Checklist

General

• My code follows the project's coding standards
• I have read and understood the CONTRIBUTING.md guide
• My changes generate no new compiler warnings
• My changes generate no new linting errors

Code Quality

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added or updated docstrings for new functions
• I have updated README if adding new features

Testing

• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes
• I have tested edge cases and error conditions
• I have verified that existing functionality is not broken

Git

• My commit messages follow the Conventional Commits format
• My branch is up to date with the main branch
• I have squashed my commits into logical units (if needed)

Security

• My changes do not introduce security vulnerabilities
• I have not hardcoded any secrets or credentials
• Sensitive data is properly handled and logged as [REDACTED]

Reviewers

• I have requested review from @maintainers
• I am available to answer questions about my changes
• I understand that changes may require additional iterations

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Additional Notes

Breaking Changes

Migration Steps

Screenshots

Related PRs

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Release Notes

## [Unreleased]

### Added
- Description of new feature

### Changed
- Description of change

### Fixed
- Description of fix

### Security
- Security-related changes

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Submitting Your PR

1. Ensure all checklist items are complete
2. Request review from maintainers
3. Respond to feedback and make necessary changes
4. Once approved, a maintainer will merge your PR

Thank you for contributing to Attest!