[codegen] graphql config - multiple extension outputs from a project

**Is your feature request related to a problem? Please describe.**
I want to output typescript types to lambda functions as well as my frontend application also.

**Describe the solution you'd like**
The ability to have many extensions with the same key as `amplify` within the same project:
```yml
projects:
Workspaces:
schemaPath: src/graphql/Workspaces/schema.json
includes:
- src/graphql/Workspaces/**/*.ts
excludes:
- ./amplify/**
extensions:
amplify:
codeGenTarget: typescript
generatedFileName: src/utils/API.ts
docsFilePath: src/graphql/Workspaces
maxDepth: 2
amplify:
codeGenTarget: typescript
generatedFileName: amplify/backend/foobar/API.ts
docsFilePath: amplify/backend/foobar
maxDepth: 2
extensions:
amplify:
version: 3
```

**Describe alternatives you've considered**
Having a folder in the root directory to allow types to be read from there, however it's not easy and requires extra permissions and npm won't let you read outside the main dir it's being ran from. So this will be an issue for lambda functions.