Add dictionary-related AST node types and EXPLAIN output

[kyleconroy]

This change adds proper support for CREATE DICTIONARY parsing and EXPLAIN AST output:

• Add new AST types: DictionaryAttributeDeclaration, DictionaryDefinition,
  DictionarySource, KeyValuePair, DictionaryLifetime, DictionaryLayout,
  DictionaryRange
• Implement dictionary parsing in parseCreateDictionary() with support for:
  • Dictionary attributes (column definitions)
  • PRIMARY KEY clause
  • SOURCE clause with key-value pairs
  • LIFETIME clause
  • LAYOUT clause
  • RANGE clause
• Add EXPLAIN output for dictionary nodes matching ClickHouse's format:
  • DictionaryAttributeDeclaration for attribute definitions
  • FunctionWithKeyValueArguments for SOURCE (with extra space before name)
  • pair (lowercase) for key-value pairs
  • Dictionary lifetime/layout/range sections
• Update metadata.json files to remove explain_todo entries for now-passing
  dictionary tests