add throttler to gql sample to illustrate bug reading 'header' of undefined

[sadams]

Reproduce:

1. run npm install
2. run npm start in sample/23-graphql-code-first
3. visit http://localhost:3000/graphql
4. execute query {recipes{title}}
5. see output:

[Nest] 95109  - 31/03/2023, 09:21:31   ERROR [ExceptionsHandler] Cannot read properties of undefined (reading 'header')
TypeError: Cannot read properties of undefined (reading 'header')
    at GqlThrottlerGuard.handleRequest (/[...]/sample/23-graphql-code-first/node_modules/@nestjs/throttler/src/throttler.guard.ts:88:9)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
asdf
[Nest] 95109  - 31/03/2023, 09:21:32   ERROR [ExceptionsHandler] Cannot read properties of undefined (reading 'header')
TypeError: Cannot read properties of undefined (reading 'header')
    at GqlThrottlerGuard.handleRequest (/[...]/sample/23-graphql-code-first/node_modules/@nestjs/throttler/src/throttler.guard.ts:88:9)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)