feat(catalog, schema): Add existence check and schema improvements

[Eyizoha]

Purpose

• feat(catalog): Support db/tbl exists check
  • Add methods to Catalog interface for checking database and table existence
• refactor(schema): Use TableSchema directly
  • Refactor Schema interface implementation to directly inherit the interface using TableSchema, similar to FileSystemCatalog and other classes, avoiding one layer of indirection overhead
• feat(schema): Add GetJsonSchema for Schema interface
  • Add method to Schema interface for retrieving schema in JSON format, which can be conveniently used in places like ReadContextBuilder::SetTableSchema

Tests

None

API and Format

Three new interface methods added to API, no impact on existing interfaces or format

Documentation

No new features introduced

[lxy-9602]

+1