feat: Add lockfile attestor

[fkautz]

This commit introduces a new lockfiles attestor to capture and attest the contents of common lockfiles in the project. The changes include:

• Add new file attestation/lockfiles/lockfiles.go implementing the lockfiles attestor
• Update imports.go to include the new lockfiles package

The lockfiles attestor captures contents of various lockfiles such as Gemfile.lock, package-lock.json, yarn.lock, and others. It stores the information in a slice of LockfileInfo structs, allowing for flexible handling of multiple lockfiles.

This feature enhances the project's capability to track and verify dependency information as part of the attestation process."