feat: Image / Index by Matcher

[dtrudg]

Allow an Image or ImageIndex to be retrieved from a SIF file by Matcher - e.g. match.Name(..) to match by ref.name annotation.

Fixes #82

[tri-adam]

Should we support retrieving images/indexes by properties other than the org.opencontainers.image.ref.name annotation?

A few potential use cases I can foresee:

1. Retrieve by digest. The current API supports this by supplying a name.Digest, but this attempts to match the org.opencontainers.image.ref.name annotation rather than the digest in the descriptor?
2. Retrieve an image from a multi-arch index. If I have a root index that points to a multi-arch index, should I be able to retrieve an image by supplying the reference (of the index) and the platform (of the image)?
3. Retrieve by artifact type. For example, in a SIF with an OCI artifact and then other artifacts that reference it, I might want to retrieve the "base" artifact by its type?

[dtrudg]

Should we support retrieving images/indexes by properties other than the org.opencontainers.image.ref.name annotation?

A few potential use cases I can foresee:

1. Retrieve by digest. The current API supports this by supplying a `name.Digest`, but this attempts to match the `org.opencontainers.image.ref.name` annotation rather than the digest in the descriptor?

2. Retrieve an image from a multi-arch index. If I have a root index that points to a multi-arch index, should I be able to retrieve an image by supplying the reference (of the index) and the platform (of the image)?

3. Retrieve by artifact type. For example, in a SIF with an OCI artifact and then other artifacts that reference it, I might want to retrieve the "base" artifact by its type?

Rewritten so the functions take the match.Matcher directly - which should allow 1, 2 directly with match.Digests / match.Platform. 3 should be possible with a custom matcher, which we could provide in oci-tools at a later date?

Look okay to you?

[dtrudg]

Should we support retrieving images/indexes by properties other than the org.opencontainers.image.ref.name annotation?
A few potential use cases I can foresee:

1. Retrieve by digest. The current API supports this by supplying a `name.Digest`, but this attempts to match the `org.opencontainers.image.ref.name` annotation rather than the digest in the descriptor?

2. Retrieve an image from a multi-arch index. If I have a root index that points to a multi-arch index, should I be able to retrieve an image by supplying the reference (of the index) and the platform (of the image)?

3. Retrieve by artifact type. For example, in a SIF with an OCI artifact and then other artifacts that reference it, I might want to retrieve the "base" artifact by its type?

Rewritten so the functions take the match.Matcher directly - which should allow 1, 2 directly with match.Digests / match.Platform. 3 should be possible with a custom matcher, which we could provide in oci-tools at a later date?

Look okay to you?

Ahh sorry - yeah 2. is nested index so this doesn't work directly, as you pointed out in Slack.

[tri-adam]

Alright, as discussed in huddle... let's take a match.Matcher (to select the top level manifest) and then functional options (as a placeholder for future logic that operates on the selected manifest, for example a platforms flag.)