AWS, GCP: Fix double-checked-locking pattern in S3FileIO, GCSFileIO.

[ChaladiMohanVamsi]

Fix: double-checked-locking pattern in S3FileIO, GCSFileIO.

Ensure Thread-Safe Initialization of clientByPrefix and storageByPrefix

This PR addresses a thread-safety issue during the lazy initialization of the clientByPrefix map. Previously, the shared map was being assigned before it was fully populated, which could lead to race conditions and visibility issues in a concurrent environment.

Changes:

• Introduced a local variable localClientByPrefix inside the synchronized block.
• Populated this local map with all required entries.
• Only after full initialization, assigned it to the shared clientByPrefix field.

Benefits:

• Prevents other threads from observing a partially initialized map.
• Eliminates the risk of NullPointerException due to premature assignment.
• Aligns with best practices for safe publication in multi-threaded contexts.

- this.clientByPrefix = Maps.newHashMap();
+ Map<String, PrefixedS3Client> localClientByPrefix = Maps.newHashMap();
...
+ this.clientByPrefix = localClientByPrefix;