Implement conventional labels on top of existing label infrastructure

[baronfel]

Labels

There are some well-known labels that we should consider setting by default:

Annotation Key	Derived from
org.opencontainers.image.created	$([System.DateTime]::UtcNow.ToString("o"))
org.opencontainers.image.description	$(Description)
org.opencontainers.image.authors	$(Authors)
org.opencontainers.image.url	$(PackageProjectUrl) if present.
org.opencontainers.image.documentation	Since $(PackageLicenseUrl) is deprecated, we may just have to default to $(RepositoryUrl). Users should be able to override this easily though.
org.opencontainers.image.version	$(Version) if present. Note that there's not really a consistent place to hook into where we can be assured of a version being set, so we'll want to be sometime directly before Build, but as late as possible otherwise.
org.opencontainers.image.vendor	There's no $(PackageOwners) or $(Owners) to bootstrap off of, but maybe we should make one?
org.opencontainers.image.licenses	$(PackageLicenseExpression)
org.opencontainers.image.title	$(Title)
org.opencontainers.image.base.digest	The resolved digest of $(ContainerBaseImage). We don't calculate this yet so can't do it.
org.opencontainers.image.base.name	$(ContainerBaseImage)
org.opencontainers.image.source	$(PrivateRepositoryUrl) if present and $(PublishRepositoryUrl) is true.
org.opencontainers.image.revision	$(RepositoryCommit) if present and $(PublishRepositoryUrl) is true.