extend blobhandler context/registration with artifact type

[mandelsoft]

What this PR does / why we need it:

A blob handler is responsible for storing local blobs in an alternate way.
For example, in a OCI registry backend for OCM repositories, local blobs describing an OCI artifact are exploded and stored as regular OCI artifacts, again. The access spec in the CD is adapted accordingly.

Therefore a blob handler gets a storage context describing the technical target repository and the blob (including its mime type). It then decides whether it will handle the blob or not. If yes, it has to return an appropriate access spec usable to access the content.

Similar is the registration, handlers are registered for implementation repositories types and mime types.
Both is optional. Even if a registration constraint matches, the handler may reject the handling of a given blob.
To solve this all matching handler are called in a prioritized order until a handler feels responsible.

All this is sufficient, to support backing technologies to provide appropriate blobs in their native formats (like the OCI artifacts) if transported content reaches it.

To support more specific handlers with the plugin concept, it would be useful to support matches for dedicated artifact types, also. This would enable automatically upload imported content directly to intended target repositories (not equal to the OCM repository) (for example a maven repo GAV content could be propagated to a local Maven repository. This is introduced with this PR, in preparation for the connections with plugins.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note: