Support POSTGRESQL dialect

[kenkania]

Wrench only supports GOOGLESQL dialect today. It fails during schema migration table creation because postgres doesn't support that syntax. It's also possible that use of memefish may cause issues (e.g. comment removal), but I haven't see issues with that in practice. You can workaround this by just precreating the migration table in a postgres-compliant way.

Can I put up a PR to detect the dialect via API (but can force dialect with flag) and use a compliant CREATE TABLE statement?

[apstndb]

GoogleSQL and PostgreSQL is completely different dialect, so I believe it shouldn't be supported without native support of PostgreSQL dialect.

I think it may be useful to implement this feature. simple parser of go-sql-spanner can support the both of GoogleSQL and PostgreSQL.
googleapis/go-sql-spanner#513