Add optional CORS header middleware

[bgervan]

Optional CORS header middleware with configs like:

[cors] enabled = true allowed_origins = ["example.com"]

or with environment variable like (in docker compose for example):

  app:
    <<: *app-defaults
    container_name: listmonk_app
    command: [ sh, -c, "yes | ./listmonk --config config.toml" ]
    depends_on:
      - db
    volumes:
      - ./config.toml:/listmonk/config.toml
    environment:
      - LISTMONK_cors__enabled=true
      - LISTMONK_cors__allowed_origins__0=*
      - LISTMONK_cors__allowed_origins__1=http://localhost:3000
      - LISTMONK_cors__allowed_origins__2=google.com
      - LISTMONK_cors__allowed_origins__2=another.com

Note: the google.com is overriden with another.com in this example.

With enabled false, everything works as before, so it doesn't break backward the compatibility. Without array of origins, it shows a warning, but works like cors disabled.

Inspiration for this PR: https://github.com/knadh/listmonk/issues/1521