Add F312 MERGE statement support

[TedCraft]

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You have an error in your SQL syntax: MERGE INTO agents a USING (SELECT * FROM agents WHERE commission > 0.15) cd ON (a.agent_code = cd.agent_code) WHEN MATCHED THEN UPDATE SET agent_code = cd.agent_code WHEN NOT MATCHED THEN INSERT (agent_code) VALUES (cd.agent_code) no viable alternative at input 'MERGE' at line 1 position 1 near @01:5='MERGE'<340>1:1

[makssent]

class org.apache.shardingsphere.sql.parser.sql.common.segment.generic.AliasSegment cannot be cast to class org.apache.shardingsphere.sql.parser.sql.common.segment.generic.table.TableSegment (org.apache.shardingsphere.sql.parser.sql.common.segment.generic.AliasSegment and org.apache.shardingsphere.sql.parser.sql.common.segment.generic.table.TableSegment are in unnamed module of loader 'app')