Use ArrayAccessor in GenericResourceOwner

[AndreiIgna]

Allow the resourceOwnerId to be specified in more formats, like user.id.
This would enable the GenericProvider to be used with more OAuth APIs.

Slack, for example, returns the user info in a nested array https://api.slack.com/methods/users.identity#response

By accepting this PR, the Slack OAuth and others can be used with the GenericProvider.

[AndreiIgna]

Second commit adds GenericResourceOwner::getAttribute() helper method, which makes it easier to access user data.

// at the moment
$userData = $resourceOwner->toArray();
$userField = $userData['field'];
$userField2 = $userData['group']['field'];

// with getAttribute() method
$userField = $resourceOwner->getAttribute('field');
$userField2 = $resourceOwner->getAttribute('group.field');

[codecov-io]

Codecov Report

Merging #788 (1fb4158) into master (77def43) will decrease coverage by 0.42%.
The diff coverage is 33.33%.

@@              Coverage Diff              @@
##              master     #788      +/-   ##
=============================================
- Coverage     100.00%   99.57%   -0.43%     
- Complexity       180      181       +1     
=============================================
  Files             20       20              
  Lines            464      466       +2     
=============================================
  Hits             464      464              
- Misses             0        2       +2     

Impacted Files	Coverage Δ
src/Provider/GenericResourceOwner.php	77.77% <33.33%> (-22.23%)	⬇️