Allow Nullable types in MySQL, ODBC and MongoDB external sources

[alexey-milovidov]

#3284

[alexey-milovidov]

Example:

CREATE TABLE mysql_information_schema_tables (
  `TABLE_CATALOG` Nullable(String),
  `TABLE_SCHEMA` Nullable(String),
  `TABLE_NAME` Nullable(String),
  `TABLE_TYPE` Nullable(String),
  `ENGINE` Nullable(String),
  `VERSION` Nullable(UInt64),
  `ROW_FORMAT` Nullable(String),
  `TABLE_ROWS` Nullable(UInt64),
  `AVG_ROW_LENGTH` Nullable(UInt64),
  `DATA_LENGTH` Nullable(UInt64),
  `MAX_DATA_LENGTH` Nullable(UInt64),
  `INDEX_LENGTH` Nullable(UInt64),
  `DATA_FREE` Nullable(UInt64),
  `AUTO_INCREMENT` Nullable(UInt64),
  `CREATE_TIME` Nullable(DateTime),
  `UPDATE_TIME` Nullable(DateTime),
  `CHECK_TIME` Nullable(DateTime),
  `TABLE_COLLATION` Nullable(String),
  `CHECKSUM` Nullable(UInt64),
  `CREATE_OPTIONS` Nullable(String),
  `TABLE_COMMENT` Nullable(String)
) ENGINE = MySQL('127.0.0.1:3306', INFORMATION_SCHEMA, TABLES, 'root', 'my-secret-pw')

SELECT * FROM mysql_information_schema_tables;

[alesapin]

LGTM

[alexey-milovidov]

TODO: Integration test.