Implement interface for enrichments

[aguschin]

To provide something like

$ gto describe iris --version v1
# Output can include DVC metrics, params, plots, dependencies, etc.
# Output can also include MLEM types, frameworks, methods, etc.
# Future output could include LDB data.

we need to implement modules like mlem.gto and dvc.gto to be later automatically discovered by GTO and used to fetch all details about artifacts.

To implement them, we first need to implement abstract interface they must satisfy.

[aguschin]

While using artifact registry, it could be the case when artifact itself was updated and enrichments are no longer correct.
$ gto describe should warn a user about these cases. It could be good if we could include this in #52 to get at least some idea how it should be done.

[aguschin]

As mentioned by @shcheklein, when DVC is installed as binary, GTO may not be able to discover DVC Python API. We need to support CLI as a fallback then. As an option, we can list dvc and mlem in config file gto.yaml so GTO will know what to search for.

[aguschin]

@mike0sv have created a good PR #52 that showcase how this should work. It's decided to postpone this functionality and implement it for the next product release.

[aguschin]

As a part of this task, we need to think about how to show discovered models in Studio MR. How it will be handled? How we'll show MLEM-only models there? Do you hide a MLEM model once GTO annotation matches the path (including previous commits)?