Support SCHEMA DDL statements

[apstndb]

This PR implements SCHEMA DDL statements, CREATE SCHEMA and DROP SCHEMA.

Note

The official docs says DROP SCHEMA can target multiple schema names.

DROP SCHEMA schema_name [, ...]

but, it seems not to be true.

spanner> DROP SCHEMA schema1, schema2;
ERROR: rpc error: code = InvalidArgument desc = Error parsing Spanner DDL statement: DROP SCHEMA schema1, schema2 : Syntax error on line 1, column 20: Expecting 'EOF' but found ','

• part of Support named schema #100

[makenowjust]

LGTM