services/pcloud: add digest auth, recursive listing, and read-path caching

[mro68]

Summary

This PR improves OpenDAL's pCloud service for real-world rustic backup workloads.

It addresses authentication compatibility issues and significantly reduces repository scan/read latency by using pCloud-native capabilities and lightweight caches.

Problem

• Username/password auth failed for pCloud accounts that require digest-based login semantics.
• Listing large repository trees was slow due to fallback traversal behavior.
• Repeated reads were paying avoidable path-resolution and link-generation costs.

Root Cause

• pCloud service auth path did not use digest authentication flow.
• The service did not expose native recursive listing capability and needed robust recursive metadata flattening.
• Read path lacked caching for stable object identifiers and short-lived direct links.

Changes

1) Digest authentication

• Use pCloud digest flow (getdigest + passworddigest) in service auth requests.

2) Native recursive listing

• Advertise recursive listing capability and implement recursive flattening for nested metadata.
• Reconstruct missing child paths from parent metadata where pCloud omits path in recursive responses.
• Keep absolute path propagation correct across recursive traversal.

3) Read-path performance caching

• Cache path -> fileid from list/stat metadata.
• Cache getfilelink results with expiration handling.
• Prefer getfilelink(fileid=...) when available.
• Invalidate caches on mutating operations (write/delete/rename/copy).

Validation

• Unit tests:
  • cargo test -p opendal-service-pcloud --lib
• Build:
  • cargo build --bin rustic
• Real-world verification with pCloud-backed rustic repository:
  • repoinfo succeeds with digest-authenticated backend access.
  • --dry-run prune succeeds and no longer shows prior path/ID parsing failures.

Downstream Impact

• rustic_core feature-enablement PR depends on these pCloud service improvements and should consume them via an OpenDAL release that includes this change set.

Notes

• Changes are intentionally limited to the pCloud service implementation.
• No API-breaking changes are introduced.