[workloadmeta] Add ContainerImageMetadata Kind

[davidor]

What does this PR do?

Adds a new Kind to the workloadmeta store: ContainerImageMetadata.

The workloadmeta collectors will be adapted to extract container image metadata and will store them in workloadmeta, but that will come in future PRs.

Describe how to test/QA your changes

Nothing for now. We will test the changes in the workloadmeta collectors that will come in future PRs.

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[clamoriniere]

maybe we can name the kind container_image
because we are only in workloadmetadata, so "metadata" is a bit redundant.
WDYT?

[davidor]

The ContainerImage struct already exists. It's the one that we use to attach basic image info to containers:

datadog-agent/pkg/workloadmeta/types.go

Line 246 in e943e52

type ContainerImage struct {