backend/opendal: enable pcloud backend feature

[mro68]

Summary

Enable OpenDAL pCloud support in rustic_core so pCloud repositories can be used natively through the OpenDAL backend.

Motivation

rustic_core already supports multiple OpenDAL-backed services. This change enables pCloud in the backend feature set to make pCloud repositories usable without external workarounds.

Changes

• Enable the pCloud service feature in backend OpenDAL dependency configuration.

Dependency / Release Note (Important)

• This PR depends on the corresponding OpenDAL pCloud fixes and performance improvements:
  • services/pcloud: add digest auth, recursive listing, and read-path caching apache/opendal#7617
• For production usage in rustic/rustic_core, this should be merged together with (or after) an OpenDAL release that includes those changes.

Validation

• Compiles and links with the updated OpenDAL pCloud implementation.
• Rebased onto current main and conflict-resolved in crates/backend/Cargo.toml (keeping upstream opendal = 0.56.0 updates while adding services-pcloud).
• cargo check -p rustic_backend --features opendal passes.

Notes

• This PR is intentionally minimal and scoped to feature enablement in rustic_core.
• Runtime behavior improvements are implemented in the corresponding OpenDAL PR.

[aawsome]

LGTM!
@mro68 Can you please add an example to
https://github.com/rustic-rs/rustic/tree/main/config/services
once rustic uses the current rusic_core version?