Allow GraphQL directive to be added

[dlhck]

Migrated from #1151

Is your feature request related to a problem? Please describe.
I would like to create GraphQL directives to add reusable validation rules to the resolvers of some plugins.

Describe the solution you'd like
Add a new optional input field when creating a plugin

@VendurePlugin({
  imports: [PluginCommonModule],
  shopApiExtensions: {
    schema: gql` ... `,
    resolvers: [/* ... */, MyCustomMutationResultResolver],
    schemaDirectives: {
       upper: UpperCaseDirective,
   }
  }
})
export class MyPlugin {}

Additional context
The current alternative is to write a lot of code to check each field and throw an error.