feat: allow DDL with autocommit=false

[olavloite]

Adds support for running DDL statements when a connection is in autocommit=false mode. By default, DDL statements are only allowed when no transaction is active. That is; no query or DML statement has been executed which activated a read/write transaction.

A new flag is added that can be used to revert the behavior back to the original behavior where DDL is always refused when autocommit=false. The same flag can also be used to make the API behave the same as MySQL and Oracle, where any active transaction is automatically committed whenever a DDL statement is encountered.

Concretely this means that the following is now allowed:

set autocommit=false; create table Singers (SingerId INT64, Name STRING(MAX)) PRIMARY KEY (SingerId); 

The following is by default NOT allowed, unless
ddlInTransactionMode=AUTO_COMMIT_TRANSACTION

set autocommit=false; select * from singers; -- This starts a transaction create table Albums (AlbumId INT64) PRIMARY KEY (AlbumId); -- This is not allowed