Install schema tester plugin using entry points

[drdavella]

Also make its use opt-in, both with a command-line option and a configuration option.

[drdavella]

Unfortunately this is going to lead to a bit of a discontinuity in compatible versions. This is because pytest will crash if a plugin is installed as an entry point and is also explicitly referenced in a project's conftest.py file. I can't seem to find a good way to provide a workaround either.

[drdavella]

It turns out that packaging the plugin in this way is proving to be unreasonably burdensome. Doing it in a stable/reliable way would require moving the plugin to a separate package, which I think is not the way to go right now.

I am still in favor of making the plugin explicitly opt-in, however.