Allow swappable lock identity providers

[cameron1729]

Rationale

We often see orphaned locks left in Redis. The hostname, while useful, is not surfaced prominently in AWS tooling.
Having the ability to override this and use an AWS ARN or similar alongside each lock would make it far easier to determine if a lock has been orphaned and is safe to manually release and also sets the stage for an automated solution. Hostnames are also routinely recycled, which can hide orphaned locks.

Implementing swappable identity providers allows us to choose metadata that better serves us when debugging locks.

Current state

The only current option is to wrap local_redislock\lock\redis_lock_factory inside another plugin and override methods, or to use other brittle hacks to mutate the lock value. This kind of approach is not ideal as it's fragile across upgrades and tough to test.

Implementation

Since modern Moodle has a DI container, and exposes the di_configuration hook which allows plugins to override the identity_provider binding - using DI is the cleanest approach. A param has also been added to the constructor to inject the identity service, so that if Moodle ever adds lock factories to the DI container properly, we can use DI the intended way (instead of doing a "service locator-ish" DI::get in the constructor.