Use DateTime for timestamp fields in storage types

[spydon]

Context

AnalyticsBucket (added in #1588) exposes createdAt / updatedAt as DateTime. The pre-existing storage types still expose their timestamp fields as String, so the public API is now inconsistent: some timestamps are parsed DateTime values and others are raw ISO strings the consumer has to parse themselves.

Aligning the existing types on DateTime is a breaking change, so it is deferred to v3.

Affected fields (packages/storage_client/lib/src/types.dart)

• Bucket.createdAt, Bucket.updatedAt (String → DateTime)
• FileObject.createdAt, FileObject.updatedAt, FileObject.lastAccessedAt (String? → DateTime?)
• FileObjectV2.createdAt (String → DateTime), FileObjectV2.updatedAt, FileObjectV2.lastAccessedAt (String? → DateTime?)
• PaginatedFile.createdAt, PaginatedFile.updatedAt (String? → DateTime?)

Notes

• Parse with DateTime.parse(...) in each fromJson, guarding nullable fields.
• Follow-up to feat(storage): add analytics (Iceberg) bucket CRUD #1588.