Image and feature digest pinning syntax inconsistency

Hey,

I think the behavior of Dev Container CLI for digest pinning is inconsistent between the image field and feature keys. For images, it accepts the following definition without a problem:

```
mcr.microsoft.com/devcontainers/python:1.1.10-3.12-bookworm@sha256:c5b8bd1aa0c5c56c18de49024d2d67873612107c492ad0d8946c78c495e6ff0c
```

However, when it comes to feature keys, the following definition is not valid:

```
ghcr.io/devcontainers/features/aws-cli:1.0.7@sha256:8d39e89cc98291953fb03cf7ef540dc1a7c78c3162f0eee9b61c800e4838f807
```

Feature keys require the following definition instead (which I've learned by digging into issues and finding #479 as I couldn't find any documentation on whether the features can be pinned), which omits the tag:

```
ghcr.io/devcontainers/features/aws-cli@sha256:8d39e89cc98291953fb03cf7ef540dc1a7c78c3162f0eee9b61c800e4838f807
```

If you were to pin the feature with its tag, you get the following error:

```
@devcontainers/cli 0.60.0. Node.js v18.18.2. darwin 23.4.0 arm64.
Start: Run: docker buildx version
github.com/docker/buildx v0.13.1-desktop.1 2368b2ee41873b948cb607251cec573ed00f00f9
...
Resolving Feature dependencies for 'ghcr.io/devcontainers/features/aws-cli:1.0.7@sha256:8d39e89cc98291953fb03cf7ef540dc1a7c78c3162f0eee9b61c800e4838f807'...
* Processing feature: ghcr.io/devcontainers/features/aws-cli:1.0.7@sha256:8d39e89cc98291953fb03cf7ef540dc1a7c78c3162f0eee9b61c800e4838f807
Path 'devcontainers/features/aws-cli:1.0.7' for input 'ghcr.io/devcontainers/features/aws-cli:1.0.7@sha256:8d39e89cc98291953fb03cf7ef540dc1a7c78c3162f0eee9b61c800e4838f807' failed validation. Expected path to match regex '/^[a-z0-9]+([._-][a-z0-9]+)*(\/[a-z0-9]+([._-][a-z0-9]+)*)*$/'.
Could not resolve Feature manifest for 'ghcr.io/devcontainers/features/aws-cli:1.0.7@sha256:8d39e89cc98291953fb03cf7ef540dc1a7c78c3162f0eee9b61c800e4838f807'. If necessary, provide registry credentials with 'docker login '.
```

Not being able to use the tag of the image within definitions with digests makes the implementation of automatic dependency management harder ([a Renovate discussion](https://github.com/renovatebot/renovate/discussions/28767)) and IMHO readability worse as you have no way of easily seeing which tag that feature is supposed to be. Would you consider changing this behavior?

Thanks! 🤗

Comienza reproduciendo el problema con las dos definiciones de feature en un devcontainer.json y sigue la salida de resolución de dependencias de features y validación de rutas de Dev Container CLI. Se considera completado cuando una referencia de feature puede conservar tanto su tag como su digest de forma coherente con las referencias de imagen, mientras la forma existente solo con digest sigue funcionando.

Escrito por el modelo de indexación a partir del texto del issue.