Add method to be able to override the exception context format

[halnique]

This PR extracted 1 protected methods to Illuminate\Foundation\Exceptions\Handler to override exception context format in downstream.

Currently if you need to customize context format of an exception, you would need to override the whole report method and do it there.

Adding this method allows us to customize context format of an exception by simply implementing the method in the applications Handler class. This is not a breaking change.

Example:

    // override
    protected function buildContext(Throwable $e): array
    {
        return [
            'exception' => [
                'origin' => $e,
                'context' => [
                    'common' => $this->context(),
                    'specific' => $this->exceptionContext($e),
                ],
            ],
        ];
    }