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.
close apache/drill#1615
