Refactor/object store in table

[JanKaul]

Closes #117

This PR creates an object_store field for Tabular structs to have a reference to the underlying ObjectStore. Before this was part of the catalog trait. This simplifies setting up the object_store with credentials for individual tables.

[gruuya]

Thank you, overall I think this is improves the flexibility of Catalog/Tabular objects!

That said, I still suspect that Catalogs should not have the object_store_builder field, at least not the RestCatalog (or it should be made into a trait). This is because, as clients of the catalog we really don't need to know the connection options for the object_store ahead of time (or at all)—all the adequate options should be returned within the load/create tabular responses.

In other words, we only need to know the namespace/table identifier (and warehouse/token), and the tables object_store options will come from the catalog server.

With the present changes we still need to pre-populate the ObjectStoreBuilder with the correct options, and only the bucket gets set upon finding the table location (by calling ObjectStoreBuilder::build).

[JanKaul]

You're right, the REST catalog shouldn't have an object_store_builder.

This is supposed to be a refactor PR to lay the groundwork for any further changes. I'll update the REST catalog with credential vending in a separate PR.