Add support for computed values

[jesseleite]

The idea of this PR was brought about by a support email convo. It adds support for computed values, with ability to show these as virtual fields on listings and publish forms, while preventing these values from being saved to data. This allows the user to display data (ie. from external database or API) that exists outside of Statamic (ie. a User's balance from a 3rd party service).

Usage:

User::computed('balance', function ($user, $value) {
    return (new SomeThirdPartyApi($user))->balance();
});

Collection::computed('articles', 'word_count', function ($entry, $value) {
    return word_count($entry);
});

And $value would give you the raw attribute from the yaml/database data, otherwise it'll return null just like Laravel's accessors do.

Todo:

• Add new visibility: computed field config.
• Ensure computed fields are read-only on publish forms.
• Filter out computed fields when calling $fields->values() or $fields->process()->values().
• Do not filter out computed fields when calling $fields->preProcess()->values() for create/edit forms.
• Ensure computed fields are still viewable on index/listings via $field->preProcessIndex().
• Ability to set computed field logic in service provider.
• Add $entry->computedData() (an array of all the callbacks evaluated).
• Ensure you can get computed values via $entry->value() and $entry->values().
• Merge computedData() in EntriesController@extractFromFields() around line 408?
• Test coverage.
• Documentation PR.
  • Add documentation for field visibility, computed values, and clarify data flow docs#885

Closes #6177
Closes #2226