core: refactor by splitting impl for each model

[isinyaaa]

Description

Splits the core layer implementation. Each file now handles a separate model and has its own test file.

No functional changes happened at all.

How Has This Been Tested?

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[tarilabs]

thank you @isinyaaa for this refactor, I can understand this simplifies maintenance as we've outgrown from the original scope since.

/lgtm

But before merging this kindly consider:

1. Add "refactor" in the commit title,

this way we make clear the intent is just refactoring per your PR description.
Maybe core: refactor by splitting impl for each model or similar?

2. Add in the commit message something ~like:

a quick bookmark to browse core api impl before this refactor is: https://github.com/kubeflow/model-registry/blob/969056f07f08925ec7b23f67d84cee6a11b58078/pkg/core/core.go
but the entire history can be browsed in the repo/on github

this way if we keep an handy bookmark link in the commit but of course the full history remains browsable locally and online and github.

[tarilabs]

per #371 (review)

thank you @isinyaaa

/lgtm
/approve

[google-oss-prow]

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