Allow `devcontainer.override.json` (Git ignored) to override `devcontainer.json`

Previous discussion: microsoft/vscode-remote-release#3279
Related to #22

Often we want to install Dev Container Features in containers that are not required for development but that we personally use.
For example, in my case it is[ the fish shell](https://github.com/meaningful-ooo/devcontainer-features/tree/main/src/fish) and [shell-history](https://github.com/stuartleeks/dev-container-features/tree/main/src/shell-history).

VS Code Dev Containers supports similar functionality in its own configuration (`dev.containers.defaultFeatures`), but it would be great if the spec could support this like [`compose.override.yml` of docker compose](https://docs.docker.com/compose/multiple-compose-files/merge/).