refactor(core): make SchemaManager an interface

[baiyangtx]

Purpose

SchemaManager is currently a concrete file-system-backed class. This couples callers to a specific implementation and makes it difficult for catalogs and table implementations to provide specialized schema-management behavior while keeping the rest of the codebase independent of the storage mechanism.

This PR turns SchemaManager into an interface and moves the existing implementation to FileSystemSchemaManager.

The change:

• defines the schema-management contract in SchemaManager;
• keeps the current file-system behavior unchanged in FileSystemSchemaManager;
• updates file-system-specific construction sites to instantiate FileSystemSchemaManager while keeping consumers typed against SchemaManager;
• preserves the existing SchemaManager static utility entry points by delegating them to the default implementation; and
• includes the latest schema operations, such as schema merging, rollback, and schema-path handling, in the interface contract.

This separation allows alternative or decorated schema-manager implementations to be introduced without requiring downstream table components to depend on the file-system implementation. No schema format or runtime behavior is changed by this refactoring.

Tests

• Existing SchemaManagerTest coverage continues to exercise schema creation, evolution, commit, rollback, and utility behavior through the interface and default implementation.
• Core compilation and project checks passed:

mvn -pl paimon-core -am -DskipTests compile

[baiyangtx]

@Aitozi @JingsongLi PTAL