[dx] Use the plugin on our own builds

[yoannmoinet]

Relies on:

• [feat] Add metadata configuration #166

What and why?

Would be great to dogfood ourselves.

How?

Use the plugins on ourselves.

Introducing the ADD_BUILD_PLUGINS var that will inject our own rollup-plugin to the builds of our published plugins, in order to report our own builds to Datadog.

It also introduces the concept of "basic build" which will allow us to build our @datadog/rollup-plugin, that we use to track and report on every other builds, in a very simple way, only creating what we need to use the plugin.

This way, we can use it to report on every other published packages, @datadog/rollup-plugin included 🤯

We use it in the CI's lint job and in the publish's prepack script.