Manifest: Support to manifest annotate "annotations"

Currently, `nerdctl manifest annotate` only supports annotating the manifest’s `platform` fields. e.g.:
```
# nerdctl manifest inspect ubuntu:20.04
{
"schemaVersion": 2,
"mediaType": "application/vnd.oci.image.index.v1+json",
"manifests": [
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"digest": "sha256:c664f8f86ed5a386b0a340d981b8f81714e21a8b9c73f658c4bea56aa179d54a",
"size": 424,
"annotations": {
"com.docker.official-images.bashbrew.arch": "amd64",
"org.opencontainers.image.base.name": "scratch",
"org.opencontainers.image.created": "2025-04-04T00:00:00Z",
"org.opencontainers.image.revision": "fb4da0956290c3b216a35315510f66ca9bc4a460",
"org.opencontainers.image.source": "https://git.launchpad.net/cloud-images/+oci/ubuntu-base",
"org.opencontainers.image.url": "https://hub.docker.com/_/ubuntu",
"org.opencontainers.image.version": "20.04"
},
"platform": {
"architecture": "amd64",
"os": "linux"
}
}
]
}
```
However, it cannot annotate the annotations of an image manifest. This limitation affects containerd checkpoint/restore feature that need to add CRIU-related annotations:
```
{
"schemaVersion": 2,
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"config": {
"mediaType": "application/vnd.oci.image.config.v1+json",
"digest": "sha256:e58e8cd88079c01bb6ee382d2e1fb21f7032a2486bb701d5aaabfcec385fb14f",
"size": 343
},
"layers": [
{
"mediaType": "application/vnd.oci.image.layer.v1.tar+gzip",
"digest": "sha256:04a2caa8c7551b96615111d1f3c6f8a3b00d986e65acfef437bb83cfafb9ed32",
"size": 44627
}
],
"annotations": {
"org.criu.checkpoint.container.name": "test",
"org.opencontainers.image.base.digest": "",
"org.opencontainers.image.base.name": ""
}
}
```
Let's extend `nerdctl manifest annotate` to support setting annotations, which would enable container checkpoint/restore scenarios requiring CRIU annotations.