The `userARN` is not stored in Polaris (is ignored)

### Describe the bug

Looks like [userARN](https://github.com/apache/polaris/blob/a59942a3e1350f017dfb9c4d2bed0edf6075d8ca/polaris-core/src/main/java/org/apache/polaris/core/storage/aws/AwsStorageConfigurationInfo.java#L50) is only used during [reading](https://github.com/apache/polaris/blob/a59942a3e1350f017dfb9c4d2bed0edf6075d8ca/polaris-core/src/main/java/org/apache/polaris/core/entity/CatalogEntity.java#L138), but not when it is written.
It seems like dead code in polaris-oss.

We should be able to store the `userARN` (similarly to `roleArn` and `externalId`) Now it is not possible.

### To Reproduce

_No response_

### Actual Behavior

_No response_

### Expected Behavior

_No response_

### Additional context

_No response_

### System information

_No response_

Start with polaris-core/src/main/java/org/apache/polaris/core/storage/aws/AwsStorageConfigurationInfo.java and polaris-core/src/main/java/org/apache/polaris/core/entity/CatalogEntity.java, comparing how userARN is read with how roleArn and externalId are written. Trace the persistence path and verify that userARN is stored and can be read back alongside those fields.

Written by the indexing model from the issue text.