Improve context management and dependency handling with plugin injection

Here's a in-progress patch for this. Tests are failing for some unclear reasons. Needs work.

Got it working.

Note for those implementing custom plugins with TFA: this change affects Tfa and TfaSetup instantiation. No longer do you pass in a class name, you pass in the object itself.

Before:

$context = array('uid' => '9');
$plugins = array(
  'validate' => 'MyTfaValidatePlugin',
  'fallback' => array(
    'MyTfaFallbackPlugin'
  )
);   

$tfa = new Tfa($plugins, $context);

After:

$context = array('uid' => '9');
$validate = new MyTfaValidatePlugin($context);
$plugins = array(
  'MyTfaFallbackPlugin' => new MyTfaFallbackPlugin($context);
);

$tfa = new Tfa($validate, $plugins, $context);

#2 accomplishes the goal but doesn't completely handle the example use case. Perhaps plugin modules should instantiate objects themselves so that they have control over constructor arguments.

This patch changes tfa_get_process() from the approach of #2 to calling new tfa_get_plugin() function that will return a plugin object. tfa_get_plugin() can refer to TFA API implementations for a 'callback' function or the plugin name suffixed with '_create'. This allows modules providing TFA plugins to control the instantiation and thereby control arguments and dependencies.

For example of this see tfa_test_login_create() in tfa_test.module after applying this patch.

#6 committed.

### Usage and upgrade notes

Users of TFA Basic will see support for this via #2376633: Support new TFA context injection and in the next tagged release.

If you have custom TFA plugins you may not need to make any changes at all depending on your integration with the TFA process.

The only required changes are if you create the Tfa or TfaSetup objects yourself somehow (such as during plugin configuration or custom TFA flows). If so you'll need to change the arguments to these classes to be full plugin objects instead of plugin machine names.

Additionally, if you want to instantiate your plugins yourself (in order to manage dependencies) you can register a 'callback' function in your hook_tfa_api. This function will be called during TFA setup and expect to receive a TFA plugin object back.