DRILL-6964: Implement CREATE / DROP SCHEMA commands Note: this PR only adds support for CREATE / DROP SCHEMA commands which allow to store and delete schema. Schema usage during querying the data will be covered in other PRs.
1. Added parser methods / handles to parse CREATE / DROP schema commands. 2. Added SchemaProviders classes to separate ways of schema provision (file, table function). 3. Added schema parsing using ANTLR4 (lexer, parser, visitors). 4. Added appropriate unit tests.