refactor: Split MongoDB service into new opendal-services-mongodb crate

[Aman202600]

Which issue does this PR close?

Closes #6908

This PR is part of the RFC-6828 core-split initiative.

Tracking issue: #6829

Rationale for this change

As part of RFC-6828, all storage backends are being moved out of opendal-core into separate crates to reduce core complexity, simplify maintenance, and isolate service-specific dependencies.

This PR moves the MongoDB backend into the new opendal-services workspace as required by the core-split plan.

What changes are included in this PR?

• Extracted MongoDB backend from core/services/mongodb into new crate: opendal-services/mongodb
• Converted imports to use opendal_core instead of crate
• Preserved all public APIs and existing behavior exactly as before
• Added mongodb feature flag to the root Cargo.toml
• Re-exported Mongodb and MongodbConfig from the opendal crate
• Added new opendal-services/mongodb/Cargo.toml
• Updated documentation (docs.md) for new crate path
• Ensured no registration macro (ctor) is used in the new service crate
• Cleaned up mod.rs structure into proper lib.rs

Are there any user-facing changes?

No.

This is an internal restructuring only.
All APIs, builder patterns, configuration keys, and runtime behavior remain identical.

Breaking changes?

None.