Move TContext generic from requestDidStart method to ApolloServerPlug…

…in Interface (#3525)

This commit provides a new type variable to the `ApolloServerPlugin` interface
to support the typing of the `context` variable throughout the lifecycle hooks.
Previously, `requestDidStart` was typed in such a way that the `TContext` typing
wasn't actually useful.

This fix is backwards-compatible for current implementors as it provides a default
type to the newly introduced type variable on the `ApolloServerPlugin` interface.

packages/apollo-server-plugin-base/src/index.ts

@@ -15,9 +15,9 @@ export {
   WithRequired,
 };
 
-export interface ApolloServerPlugin {
+export interface ApolloServerPlugin<TContext extends Record<string, any> = Record<string, any>> {
   serverWillStart?(service: GraphQLServiceContext): ValueOrPromise<void>;
-  requestDidStart?<TContext>(
+  requestDidStart?(
     requestContext: GraphQLRequestContext<TContext>,
   ): GraphQLRequestListener<TContext> | void;
 }