Rename "guice" extension to "inject"

[thboileau]

cf this discussion: #1382

[kerbymart]

Hi @thboileau if the extension would be renamed to "inject" instead, does it mean the module should also support other DI?

e.g.
#1411

[jlouvel]

@kerbymart Based on our PoC for Dagger2, what would you like to see in a potential org.restlet.ext.inject extension? How could the code there be adapted to work equally well for Guice.

@Tembrel who had created the org.restlet.ext.guice extension initially is wondering if this deserves to stays an extension or if it shouldn't be a set of best practices. If that was the case, we could move the code to the official example and work on it from a documentation perspective.