Summon - next iteration

[dmpetrov]

An iteration on summoning (see https://github.com/iterative/dvcx/pull/4):

1. Some minor Summon file format changes and default file name change to Summon.yaml.
2. Separating reading objects and pulling data. It is needed for flexibility and especially for data objects with several files like a compound CSV file (see in api: high level dvc.api import/export meets data catalog #2719).
3. Summon object update update_dobj(). This is not completed but it does not break current read-only logic.
4. Moving part of the logic to SummonDesc class.

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[dmpetrov]

@Suor should we close this one?