customizable db locking during migration

[fuzzbuck]

? - Why
Various pgwire databases such as CockroachDB do not support such a feature.

[iodb] running migrations failed: Execute(Database(PgDatabaseError { severity: Error, code: "42883", message: "unknown function: pg_advisory_lock()", detail: None, hint: None, position: None, where: None, schema: None, table: None, column: None, data_type: None, constraint: None, file: Some("name_resolution.go"), line: Some(205), routine: Some("ResolveFunction") }))

In such scenarios the only way to perform migrations is without locking

This PR allows the end user to choose whether to lock the DB when running migrations or not.
Default behaviour is maintained and DBs are still locked on default.

Example:
sqlx::migrate!().set_locking(false).run(&pool).await

(Changes have been tested & are actively used in development of our app)

[fuzzbuck]

Please review again

[abonander]

Can you run cargo fmt real quick?

[fuzzbuck]

there

[abonander]

The instantiation of Migrator in this doctest is missing locking: true: https://github.com/fuzzbuck/sqlx/blob/patch-1/src/macros/test.md?plain=1#L134

[fuzzbuck]

Is that what was causing the CI to fail?

Anyway, that should be good now. Perhaps I shall make a suggestion to add information about doctests here?
EDIT: as well as cargo formatting, common new contributor mistakes, etc.
https://github.com/launchbadge/sqlx/blob/main/CONTRIBUTING.md