Add a bootstrapper to allow OOMs to be handled

[imjoehaines]

Waiting on bugsnag/bugsnag-php#621 to be merged & released

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Goal

This PR adds a new OomBootstrapper which can be used to enable Bugsnag Laravel to capture OOMs. The implementation is essentially the same as bugsnag/bugsnag-php#621, but has to use a slightly different mechanism as the Bugsnag\Handler class isn't used in Bugsnag Laravel

This can be used in conjunction with the new memory_limit_increase option (see bugsnag/bugsnag-php#621) to give Bugsnag enough headroom to send OOMs

Design

The new OomBootstrapper has to be registered manually by the developer as the first bootstrapper in their HTTP kernel. This is because Laravel's shutdown function will crash before our code is called if we register a shutdown function after Laravel's

The bootstrapper doesn't depend on anything that's Laravel specific, so Lumen apps can also use it by manually calling bootstrap before creating the Lumen Application

Changeset

• New OomBootstrapper class to allow handling OOMs
• Support for the memory_limit_increase option (see Allow Bugsnag\Handler to handle OOMs bugsnag-php#621)

Testing

• Maze Runner tests that trigger OOMs added
• Unit tests for config values
• Manual testing against various Laravel/Lumen versions that are not covered in MR test matrix