Make addStage Aggregation Builder method public

[Steveb-p]

Q	A
Type	feature
BC Break	no
Fixed issues

Summary

Makes addStage method public to allow easier usage of "custom" Stage class objects.

Following discussion in https://doctrine.slack.com/archives/CAAE6T66B/p1552300833003900 / https://doctrine.slack.com/messages/CAAE6T66B/convo/CAAE6T66B-1552300833.003900/

[alcaeus]

Technically, I could merge this, but while taking another look I noticed that this only partially helps. Any stage method in the builder class returns the newly added stage, which works because the base Stage class implements all methods also implemented by the builder. This means that you can simply chain all method calls. However, since the base class doesn't offer a public addStage method, this is not chain able like other methods:

$builder
    ->match()
        ->field('foo')->equals('bar')
    ->addStage(new Match()) // <- fatal error because the match stage returned above does not have an addStage method
    ->group()
    // ...
;

I'm not sure this is a good idea, as it may be quite unexpected and confusing. I'm not sure this warrants another minor release of doctrine/mongodb that makes these methods public. With this in mind, I would suggest moving this back to 2.0 where we are in complete control of the API and can add the missing methods. What do you think @Steveb-p?

[alcaeus]

Closing in favour of #2020, we can't really add this in 1.3 without making the fluent API of aggregation builder confusing, as addStage would only be available on the builder, not on stages.

[Steveb-p]

@alcaeus Sorry I totally missed that comment in my mailbox :/ Just wanted to tell you I really appreciate your work :) Thanks for merging this to 2.0 :)

[alcaeus]

Thank you for making the initial PR - sorry we couldn't get it into 1.3. 🍻