feat: restructure pillager for improved modularity and feature expansion [TASK-10] [TASK-11] [TASK-13]

[brittonhayes]

Changes

• Upgrade from Go v1.18 to v1.21
• Rename hunter package to scanner
• Adopt plugin-style approach to scanners to enable the use of more scanners than just gitleaks
• Make better usage of interfaces to share contracts between packages and dependencies
• Decouple gitleaks datamodels from pillager data models to improve code modularity
• Redesign all report templates with a fresh look and better context
• Aggregate gitleaks rules and the pillager config into a shared pillager.toml
• Move project wide models to pillager project root for easier sharing of universal types without creating cyclic dependencies

[notion-workspace]

Add json-pretty output in addition to default json output

[notion-workspace]

Upgrade golang version to v1.21