Move TContext generic from requestDidStart method to ApolloServerPlugin Interface (#3525)
Move TContext generic from requestDidStart method to ApolloServerPlugin 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.

apollostack/apollo-server