feat: add support for Fleet media types

Sebuhi Shukurovrequested to merge
sabuhi_sh/container-registry:feat/add-fleet-media-type into master

Add support for Fleet media types (application/fleet.file and application/fleet.manifest) by inserting them into the media_types table via a pre-migration.

Related to #2155

This is needed to allow the container registry to store and serve Fleet artifacts alongside existing OCI content.

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