[BUG] - AWS_REGION mandatory - AWS STS client instantiated inside PolarisService not in PolarisCore

### Is this a possible security vulnerability?

- [X] This is NOT a possible security vulnerability

### Describe the bug

Hello,
Maybe I am wrong and this is an expected behavior.

If there is no AWS_REGION setted in the environnement where Polaris is running (inMemory), the action of creating a catalog throw an error during the initialization of the STS Client.

### To Reproduce

- Do not have AWS_REGION setted in environnement running Polaris
- start Polaris in mode "InMemory" (metaStoreManagerFactory == InMemoryPolarisMetaStoreManagerFactory)
- after Polaris is started with success
- create a catalog (with curl by example)
- STSClient fail during instantiation in polaris-service/src/main/java/org/apache/polaris/service/PolarisApplication.java line 160 (metaStoreManagerFactory)

### Actual Behavior

- In polaris-service/src/main/java/org/apache/polaris/service/PolarisApplication.java
- line 160 the PolarisStorageIntegrationProviderImpl will try to intantiate by default a AWS stsClient
- without AWS_REGION setted, it's failing

### Expected Behavior

- Maybe let the "Polaris Core" storage implementation fully manage this STS client (why is it instantiated in the "Polaris Service" ?)
- Maybe set a default Region in the STSbuilder to avoid an error ?

### Additional context

In case of a GCP or Azure deployment, AWS_REGION should not be mandatory.

### System information

Dev mode

Start in polaris-service/src/main/java/org/apache/polaris/service/PolarisApplication.java around line 160 and reproduce the catalog-creation flow in InMemory mode without AWS_REGION. Trace why PolarisStorageIntegrationProviderImpl instantiates an AWS STS client during initialization. Done means catalog creation works without AWS_REGION for InMemory deployments, including non-AWS environments, with the relevant behavior verified.

Written by the indexing model from the issue text.