Return a callable object from install()

[mortenpi]

The return value can be confusing to users, since it looks a bit like an error message:

julia> PkgAuthentication.install("juliahub.com")
#register_auth_error_handler##0 (generic function with 1 method)

So let's wrap it in an object, so we can have a more informative show representation:

julia> uninstall = PkgAuthentication.install()
PkgAuthentication.Uninstall (call this object to remove Pkg hooks)

julia> uninstall()

Also add docs to explain what this is for.

cc @ranjanan

[pfitzseb]

The return value is technically meaningful, since it's the only way to unregister the auth handler from Pkg at a later time. That said, I wouldn't mind removing that functionality from install too much.

[mortenpi]

@pfitzseb I updated it to return a callable object instead. Should be good to go if we're happy with it.