Allow context functions that directly return the context (and not a promise)
Currently, the function that constructs the context has to return a promise:
https://github.com/apollographql/apollo-server/blob/256aa937c8a7925f39b20729571c3f3a8780f055/packages/server/src/externalTypes/context.ts#L8-L11

But sometimes generating the context from the request can be done synchronously, and in this it would make sense to allow ` Promise<TContext> | TContext` as return type.