Make content extensions public again

Make content extensions public again as it has very useful reflection methods. We use them in our other projects.

@tomspilman turned this into internal around a month ago in this commit https://github.com/mono/MonoGame/commit/562ff8d654c903bf97f64962b954b4b8fad91ec7, but while the others are MonoGame specific, these are very general reflection methods, unrelated to MonoGame.