[5.x] OAuth tweaks

[jasonvarga]

This continues from #9612. The following two breaking changes are super low impact and probably only used by us in the core anyway.

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Provider::label() is now just a getter.
To configure a label, pass it in the config.

-$provider = (new Provider('twitter'))->label('X');
+$provider = new Provider('twitter', ['label' => 'X']);

The way a user would define the label is within the config file - not in PHP - and that doesn't change at all.

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User/UserRepository::findByOAuthId() now accepts a provider instance rather than the name string.

-$user = User::findByOAuthId('twitter', 7);
+$provider = OAuth::provider('twitter');
+$user = User::findByOAuthId($provider, 7);

This allows us to avoid re-fetching the provider instance. Probably the only place this is ever called is from within the core where we are already in that provider instance. The config was removed from the test since it no longer has to do another lookup.