feat: add secret scanning

[knqyf263]

Description

It detects secrets such as AWS Access Key and GitHub PAT.

Issues

• https://github.com/aquasecurity/trivy/issues/1950

TODO

• [x] Add secret analyzer
• [x] Add secret scanner
  • [x] Support secret detection
  • [x] Support line numbers
  • [x] Mask secrets
  • [x] Support custom rules from a config file
• [x] Add built-in rules (https://github.com/aquasecurity/fanal/pull/464)
• [x] Support allow patterns (#443)
• [x] Support keywords (#472)
• [x] Optimization
  • [x] Skip some files, dirs and extensions
  • [x] Skip binaries
  • [x] Consider container image scanning
• [x] Add unit tests
  • [x] secret/scanner.go
  • [x] analyzer/secret/secret.go
• [x] Add integration tests (will be added in Trivy)