StorageConfigInfo should be optional for EXTERNAL catalogs

### Describe the bug

Currently, StorageConfigInfo is required for both INTERNAL and EXTERNAL catalogs.

However for federated catalogs, Polaris does not need this information since it can access it from the source of truth catalog. To fix this issue we need to change the API spec as well as Polaris Server validations during CreateCatalog requests.

Once that is done, we no longer need to set the `ALLOW_OVERLAPPING_CATALOG_URLS` in the regression/integration tests for catalog federation.

### To Reproduce

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### Actual Behavior

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### Expected Behavior

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### Additional context

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### System information

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Start by locating the API specification and Polaris Server's CreateCatalog validation for StorageConfigInfo, then inspect the regression and integration tests for catalog federation. Done means StorageConfigInfo is optional for EXTERNAL catalogs while remaining required for INTERNAL catalogs, and the federation tests no longer need ALLOW_OVERLAPPING_CATALOG_URLS.

Written by the indexing model from the issue text.