Make ApolloEngineLoader optional

**Is your feature request related to a problem? Please describe.**

`@graphql-toolkit/apollo-engine-loader` depends on `apollo-language-server` which is a huge dependency. But we are not using Apollo Server and we do not need this feature.

**Describe the solution you'd like**

The dependency tree includes a lot of modules of Apollo Server that do not seem to relate to GraphQL code generation, such as `apollo-link`. It would make more sense to make the loaders into plugins.

**Describe alternatives you've considered**

It should also be possible to make `apollo-engine-loader` more lightweight so it doesn't depend on `apollo-language-server`.

**Additional context**

```
$ yarn why apollo-language-server
yarn why v1.21.1
[1/4] 🤔 Why do we have the module "apollo-language-server"...?
[2/4] 🚚 Initialising dependency graph...
[3/4] 🔍 Finding dependency...
[4/4] 🚡 Calculating file sizes...
=> Found "apollo-language-server@1.17.2"
info Reasons this module exists
- "@graphql-codegen#cli#@graphql-toolkit#apollo-engine-loader" depends on it
- Hoisted from "@graphql-codegen#cli#@graphql-toolkit#apollo-engine-loader#apollo-language-server"
info Disk size without dependencies: "8.8MB"
info Disk size with unique dependencies: "42.48MB"
info Disk size with transitive dependencies: "8.8MB"
info Number of shared dependencies: 59
✨ Done in 1.29s.
```