fix: added OCI Image Index mediaType

[mdaizcorbe]

Description

Added the OCI Image Index's mediaType for completeness sake per the OCI spec and to work around a known Sonatype Nexus Repository bug.

Related Issue

Fixes #2351

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Other (security config, docs update, etc)

Checklist before merging

• Test, docs, adr added or updated as needed
• Contributor Guide Steps followed

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[Noxsios]

@mdaizcorbe Thanks for the contribution!

@AustinAbro321 also noticed this in his current OCI library refactor work: #2235.

If you have the time, are you able to verify this fixes Nexus?

[Racer159]

lgtm

[mdaizcorbe]

@mdaizcorbe Thanks for the contribution!

@AustinAbro321 also noticed this in his current OCI library refactor work: #2235.

If you have the time, are you able to verify this fixes Nexus?

Yes, I reproduced the issue on a Nexus Repo container instance running locally using the current release of Zarf CLI. Then rebuilt the Zarf CLI with the one line change and all OCI artifacts persisted after several manual runs of the Nexus Repo cleanup task.