Caching requests while authenticating as an Integration

[PhiloNL]

Not sure if this is a bug or something which should be added to the documentation.
When caching requests the cache key is based on a couple of request headers (#558). I'm assuming this works fine when using AUTH_ACCESS_TOKEN for authentication as this token does not change.

When authenticating as an integration it's required to include a JSON Web Token which is valid for a few seconds up to a couple of minutes (I think 10 minutes is the limit). I'm always including a new JWT token which works fine but I'm guessing this is causing trouble with caching since the cache key is changing on every request because the Authorization header has changed (new JWT token).

Replacing the Authorization header on the HeaderCacheKeyGenerator with the installation id seems like a good alternative:

        $builder = new Builder();
        $builder->addHeaderValue('Installation-id', $installationId);

        $client = new Client($builder, 'machine-man-preview');
        $client->addCache($pool, [
                'cache_key_generator' => new HeaderCacheKeyGenerator([
                    'Installation-id',
                    'Cookie',
                    'Accept',
                    'Content-type',
                ]),
            ]
        );

Not sure if there is a better alternative to fix this.
Let me know if you want me to create PR to update the documentation.