support registry mirrors for image scanning

[itaysk]

Discussed in #4957

When scanning images, Trivy is pulling the container image from registry directly (using OCI API). Most container engines support "mirroring", which tells the engine to pull certain images from another registry instead of the user specified.
Example from Docker: https://docs.docker.com/docker-hub/mirror/#configure-the-docker-daemon

Trivy should support similar functionality for pulling images from a mirror if configured. Since mirror configuration varies between engines, we should not support one of the existing ones but allow users to configure specifically for Trivy.

As @caleb-devops suggested:

registry-mirrors:
  - src: "docker.io"
    dest: "harbor.example.com/docker.io"
  - src: "ghcr.io"
    dest: "harbor.example.com/ghcr.io"

it could be supported on cli as --registry-mirrors docker.io:harbor.example.com/docker.io, but since this is advanced use case it might be ok to keep it config file exclusive.