Make recommended changes to our module manifests (breaking change for v0.23)

[StephenWeatherford]

Definitely do:

1. allow a non-empty config and an unrecognized config media type when we download a module (currently we throw)
2. allow multiple module layers when we download (currently we throw)

Consider (would break previous Bicep versions from being able to download newly-published modules, including those in the public registry):

1. Write "{}" as the config contents
2. Change config media type to application/vnd.oci.empty.v1+json?
3. We could consider a phase out with warning approach...

Context:

I discovered that ACR requires the sources manifest config to be valid JSON or it won't recognize it as an attachment. Upon asking the ACR guys if this was expected, I got this answer:

Hey Stephen, empty manifest support is somewhat inconsistent industry wide. Configs have only recently become that free as of the most recent RC3 image spec and we have not yet implemented support for that at the config level. I'll make sure to add it as a workitem nonetheless, thanks for bringing it to our attention. Regardless we would not suggest for the use of actually empty configs or layers for that matter though since it would not be portable as different registries have different levels of support for empty layers/config. The general guidance is to use the empty descriptor https://github.com/opencontainers/image-spec/blob/main/manifest.md#guidance-for-an-empty-descriptor as you have done in the example above if no config is wanted. RC3 Also allows for the complete removal of the config field but we haven't completed support for that (Its an ongoing change one of my colleagues is working on though so that is also an option in the future). P.S did you observe this behaviour only with the empty config or also with a binary one?

[shenglol]

We will need to emit a warning about the breaking change as the first step.

[StephenWeatherford]

related branch: sw/allow-multiple-layers

[StephenWeatherford]

The ACR guys suggested the following changes:

{
"schemaVersion": 2,
"mediaType": "application/vnd.oci.image.manifest.v1+json", <<<<<<<<<<< ADDED
"artifactType": "application/vnd.ms.bicep.module.artifact",
"config": {
"mediaType": "application/vnd.ms.bicep.module.config.v1+json",
"digest": <digest of {}>, <<<<<<<<<<<< CHANGED
"size": 0, <<<<<<<<<<<<<<< CHANGED FROM 0 TO 2 (pointing to "{}" blob)
"annotations": {}
},
"layers": [
{
"mediaType": "application/vnd.ms.bicep.module.layer.v1+json",
"digest": "sha256:fa703356d827467cede2e38c00501fc9fc8f66eba3e6e13a0a3f37adae50528a",
"size": 117570,
"annotations": {}
}

[StephenWeatherford]

SUMMARY OF CHANGES

Now

• allow a non-empty config and an unrecognized config media type when we download a module (currently we throw) Be more forwards compat w/module restoring #11680
• allow multiple module layers when we download (currently we throw) Be more forwards compat w/module restoring #11680
• CANCELLED: obsolete warning?
• Change manifest media type to application/vnd.oci.image.manifest.v1+json instead of null Be more forwards compat w/module restoring #11680
• Write all layers to module cache?

Breaking change in v0.23 - see PR for for breaking change details)

• Write "{}" as the config contents instead of null/empty Write "{}" for config (breaking change, 11482) #12057

[StephenWeatherford]

See #12057 for breaking change details