Reorganize the template pkg, make it match upstream patterns

* Storage -> REST

```
$ tree pkg/template/
pkg/template/
├── api
│   ├── doc.go
│   ├── register.go
│   ├── types.go
│   ├── v1beta1
│   │   ├── register.go
│   │   └── types.go
│   └── validation
│       ├── doc.go
│       ├── validation.go
│       └── validation_test.go
├── doc.go
├── generator
│   ├── doc.go
│   ├── examples
│   │   ├── doc.go
│   │   ├── remotevalue.go
│   │   └── remotevalue_test.go
│   ├── expressionvalue.go
│   ├── expressionvalue_test.go
│   └── generator.go
├── registry
│   ├── rest.go
│   └── rest_test.go
├── template.go
└── template_test.go
```

I'm also thinking of moving TemplateProcessor to its own `pkg/template/processor/` subdirectory. And imho, generators could have subdirectories as well (`pkg/template/generator/expression/` etc.).

@soltysh @smarterclayton review pls.