[MAINTENANCE] Implement context.datasource_update with a cloud backend

[anthonyburdi]

Changes proposed in this pull request:

• Change update_by_name to update which processes a DatasourceConfig with name (populated with name in DataContext.update_datasource()`
• Leverage _build_key_from_config()

Definition of Done

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• [x] My code follows the Great Expectations style guide
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• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have added unit tests where applicable and made sure that new and existing tests are passing.
• [x] I have run any local integration tests and made sure that nothing is broken.

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