Migrator: Allow to run migration files outside of transaction for Sqlite PRAGMAs

[malteneuss]

I have found these related issues/pull requests

Relates to #2085

Description

When using Sqlite3 you need to setup some (feature) settings for your database initially like

-- 00000_initial_setup.sql
PRAGMA foreign_keys = ON;
PRAGMA journal_mode = WAL;

However, the migrator runs the whole file in a single transaction, which is correct for the remaining migration files, but leads to an error with Sqlite3:

ExecuteMigration(Database(SqliteError { code: 1, message: "Safety level may not be changed inside a transaction" }), 0)

Prefered solution

Add some (comment) marker to the migration file format that instructs the migrator to run this particular file outside of a transaction. E.g. the popular dbmate migration tool supports a feature flag "transaction" among others:

-- migrate:up transaction:false
PRAGMA foreign_keys = ON;
PRAGMA journal_mode = WAL;

Alternatively, document the workaround of #2085 (comment):

-- terminate the original migrator TRANSACTION
COMMIT TRANSACTION;

-- 00000_initial_setup.sql
PRAGMA foreign_keys = ON;
PRAGMA journal_mode = WAL;

-- restart new migrator TRANSACTION
BEGIN TRANSACTION;

Is this a breaking change? Why or why not?

No breaking change, as this would be an optional, additional feature flag.

[abonander]

-- no-transaction support was added by #4015 which is available in the 0.9.0-alpha.1 release. See the CHANGELOG for details: https://github.com/launchbadge/sqlx/blob/2a1eedd29962ce81a581f858337016edac160100/CHANGELOG.md