Tune container image fields to align with Otel SemConv

[ChrsMark]

This PR tunes the container.image.* fields to align with Otel's SemConv.
Specifically after open-telemetry/semantic-conventions#159 have been merged, we need to adjust ECS to at least:

1. container.image.tag -> container.image.tags
2. container.image.hash.all -> container.image.repo_digests

For now I have added container.image.repo_digests as a new field and we can keep container.image.hash.all for backwards compatibility if we prefer.

---

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[github-actions]

Documentation changes preview: https://ecs_2282.docs-preview.app.elstc.co/diff

[github-actions]

This PR is stale because it has been open for 60 days with no activity.

[ChrsMark]

cc: @elastic/ecs

[norrietaylor]

cc/ @trisch-me @mjwolf

[ChrsMark]

Closing for now since the plan is not to update ECS back with changes from SemConv.