Service: monoiofs #6829

[codxbrexx]

Which issue does this PR close?

Closes #6829.

Rationale for this change

This PR extracts the monoiofs service from opendal-core into its own crate opendal-service-monoiofs as part of the core-split effort (RFC-6828). This improves modularity and compilation times by removing service-specific dependencies (like monoio, flume) from the core crate.

What changes are included in this PR?

• Created a new crate core/services/monoiofs.
• Moved all monoiofs source code from core/core/src/services/monoiofs to the new crate.
• Removed monoiofs source and dependencies (monoio, flume) from opendal-core.
• Updated opendal-core to remove the internal monoiofs module.
• Updated the opendal facade to re-export Monoiofs from the new crate when the services-monoiofs feature is enabled.
• Ensures all imports use opendal_core instead of crate.

Are there any user-facing changes?

No. The public API opendal::services::Monoiofs remains exactly the same for users of the library. Internally, the code location and dependency structure have changed.

AI Usage Statement

No.