support CREATE VIEW

[jennifersp]

Depends on:

• add SubStatementStr for DDL struct vitess#332

• use SubStatementStr field in DDL for getting sub statement in CREATE VIEW go-mysql-server#2448

• add parser interface in engine go-mysql-server#2486

• use parser interface in engine dolt#7819

[Hydrocharged]

LGTM! Only thing we need to add are tests that make use of some Postgres-specific syntax. One example is CREATE VIEW v AS SELECT pk::INT2 FROM t1 ORDER BY pk;. Just a few different ones, as it looks like all of the defined views are fully valid MySQL statements, so we need to guarantee that it succeeds for invalid MySQL statements that are valid PostgreSQL statements.

[Hydrocharged]

Overall looks good! Just have a big question regarding the usage of the vitess parser instead of our Postgres parser.