exportGqlSchemaPath doesn't work and pluginHook type error.
### Summary

I'm using postgraphile with apollo server and typescript. The exportGqlSchemaPath property doesn't work because when I run the code with ts-node the schema is not beign exported and when I try to run "tsc" I get a type error from "psotgraphile" package:
```
node_modules/postgraphile/build/postgraphile/pluginHook.d.ts:76:36 - error TS2344: Type 'Extra' 
does not satisfy the constraint 'Record<string, unknown>'.                                      l
  Index signature is missing in type 'Extra'.

76         context: graphqlWs.Context<GraphQLWSContextExtra>;
```

### Steps to reproduce

tsconfig.json:
```
{
  "compilerOptions": {
    "module": "commonjs",
    "declaration": true,
    "removeComments": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "allowSyntheticDefaultImports": true,
    "target": "es2017",
    "sourceMap": false,
    "outDir": "./dist",
    "baseUrl": "./",
    "incremental": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "**/*.spec.ts"]
}
```

index.ts
```
import * as pg from 'pg';
import { makeSchemaAndPlugin } from 'postgraphile-apollo-server';
import { ApolloServer } from 'apollo-server-fastify';
import { ApolloServerPluginDrainHttpServer } from 'apollo-server-core';
import { ApolloServerPlugin } from 'apollo-server-plugin-base';
import fastify, { FastifyInstance } from 'fastify';
import { PostGraphileOptions } from 'postgraphile';

const getPgPool = () =>
  new pg.Pool({
    connectionString: 'postgres://northic:northic@localhost/turnapp',
  });

const fastifyAppClosePlugin = (app: FastifyInstance): ApolloServerPlugin => {
  return {
    async serverWillStart() {
      return {
        async drainServer() {
          await app.close();
        },
      };
    },
  };
};

const startApolloServer = async () => {
  const app = fastify();

  const pgOptions: PostGraphileOptions = {
    dynamicJson: true,
    exportGqlSchemaPath: './src/generated/schema.graphql',
  };

  const { schema, plugin: pgPlugin } = await makeSchemaAndPlugin(
    getPgPool(),
    'public', // PostgreSQL schema to use
    pgOptions,
  );

  const server = new ApolloServer({
    schema,
    plugins: [
      fastifyAppClosePlugin(app),
      ApolloServerPluginDrainHttpServer({ httpServer: app.server }),
      pgPlugin,
    ],
  });

  await server.start();
  app.register(server.createHandler());
  await app.listen(4000);
  console.log(`🚀 Server ready at http://localhost:4000${server.graphqlPath}`);
};

startApolloServer();
```

### Expected results

schema file generated
typescript compiles code

### Actual results

```
node_modules/postgraphile/build/postgraphile/pluginHook.d.ts:76:36 - error TS2344: Type 'Extra' 
does not satisfy the constraint 'Record<string, unknown>'.
  Index signature is missing in type 'Extra'.

76         context: graphqlWs.Context<GraphQLWSContextExtra>;
```

### Additional Notes
postgraphile version: 4.12.8
typescript version: ^4.5.5
node version: 14.17.0
