feat(auth,storage): add session cache fast-path and storage identifier configuration

[sean-escendant]

Issue

Related: #1102, #3340

Description

This PR introduces two improvements:

1. Auth session cache fast-path: Adds an in-memory cache for fetchAuthSession() that bypasses the TaskQueue when tokens are still valid (with a 2-minute expiry buffer). This eliminates ~300-1000ms of serialization overhead during concurrent token fetches at app startup. The cache is thread-safe (NSLock) and cleared on signOut().

2. Storage identifier configuration: Adds sessionIdentifier and sharedContainerIdentifier options to the Storage plugin configuration (AmplifyOutputsData.Storage and JSON config). This enables customization of URLSession identifiers and app group shared container identifiers for background transfer and extension support.

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
  • Integration tests require AWS backend provisioning and are not applicable for this fork PR
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
  • Thread-safe cache with NSLock, cleared on sign-out, in-memory only
• Documentation update for the change if required
  • Added inline documentation for StorageConfiguration, AmplifyOutputsData.Storage properties, session cache, and areTokensValid()
• PR title conforms to conventional commit style
• New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• If breaking change, documentation/changelog update with migration instructions
  • No breaking changes — all new parameters are optional with defaults

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.