[refactor](fe) Phase 0: Prerequisite decoupling for filesystem SPI

[morningman]

Summary

This PR completes Phase 0 of the FE filesystem SPI refactoring — removing compile-time couplings that would otherwise prevent splitting filesystem implementations into independent Maven modules in later phases.

Changes

P0.1 — FsStorageType enum migration

• Introduce FsStorageType enum in fe-foundation (zero-dependency module) to replace Thrift-generated StorageBackend.StorageType in PersistentFileSystem
• Add FsStorageTypeAdapter in fe-core for bidirectional Thrift↔FsStorageType conversion
• Update all subclasses and callers: Repository, BackupJob, RestoreJob, CloudRestoreJob

P0.2 — ObjStorage IOException bridge

• Add IOException-based default bridge methods to ObjStorage interface: checkObjectExists, getObjectChecked, putObjectChecked, deleteObjectChecked, deleteObjectsChecked, copyObjectChecked, listObjectsChecked
• Add ObjStorageStatusAdapter for Status→IOException conversion (maps NOT_FOUND → FileNotFoundException)
• Zero changes to existing S3ObjStorage/AzureObjStorage implementations

P0.3 — SwitchingFileSystem decoupling

• Introduce FileSystemLookup functional interface
• Refactor SwitchingFileSystem to accept FileSystemLookup instead of ExternalMetaCacheMgr
• Update FileSystemProviderImpl to pass a lambda: key -> extMetaCacheMgr.getFsCache().getRemoteFileSystem(key)

P0.4 — MultipartUploadCapable interface

• Extract MultipartUploadCapable interface from ObjFileSystem, removing the forced abstract completeMultipartUpload method
• S3FileSystem and AzureFileSystem now explicitly implements MultipartUploadCapable
• HMSTransaction updated to use instanceof MultipartUploadCapable check instead of ObjFileSystem cast

P0.5 — GsonUtils compile-time decoupling

• Introduce FileSystemDescriptor POJO for Repository metadata serialization, replacing direct PersistentFileSystem subclass serialization
• Repository gains fs_descriptor field (new format) with backward-compatible reading of old storage field
• GsonUtils removes 7 compile-time imports of concrete filesystem classes; replaces with Class.forName() string-based reflection (graceful ClassNotFoundException handling)

P0.6 — FileSystemSpiProvider skeleton

• Add FileSystemSpiProvider interface in fs/spi/ as the future ServiceLoader contract for Phase 3 module split

Build fix

• Fix Maven build cache incorrectly skipping checkstyle:check (runAlways + include checkstyle config files in cache input)
• Fix 2 checkstyle violations introduced by this refactoring (unused import, import order)

Testing

• FE build passes: ./build.sh --fe ✅
• Checkstyle: 0 violations ✅
• All changes are backward compatible; no behavior change

Related

Closes part of #61860

[Thearas]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?