refactor(appregistry): use app registry

[julienrbrt]

Closes: https://github.com/ignite/apps/issues/94

• [x] Add install command to install apps directly from the registry
• [x] Rename registry fields to make more sense (omit empty, simplify structure)
• [x] Rename marketplace to appregistry to naming consistency
• [x] Update marketplace app to use app registry (instead of github topics)

[julienrbrt]

We quickly talked about renaming this as extensions app, or registry app, but while thinking about it, I think we should rename this to extensions if we rename apps to extensions otherwise it reads as the extensions registry app.

Additionally, while renaming registry makes sense as well, but renaming to registry could clash with the future module registry app, and naming it is extensionsregistry leads to the same problem after. Naming it marketplace avoid those two, while still leave place to a possible future renaming.