apollo-server-core: Remove inconsistent @cacheControl auto-definition

[glasser]

In Apollo Server 2, ApolloServer sometimes adds the definition of
@cacheControl to your schema.

Specifically, it adds it whenever you rely on the implicit call to
graphql-tools' makeExecutableSchema (ie, you pass typeDefs and
resolvers). It doesn't add it when you pass your own schema, and
oddly it doesn't add it when you use modules.

On the other hand, it doesn't care if you actually have cache control
enabled in your app.

What this means is that switching between the implicit call to the
built-in version of makeExecutableSchema and using your own copy of
graphql-tools has a surprising impact on your schema. We'd like to
make the implicit call just a shorthand and encourage folks who want to
use all of the different options of makeExecutableSchema to call it
directly in their app (rather than having tons of pass-through
arguments), so we don't want the semantics to differ in a surprising and
undocumented way.

Plus, other tools that look at your schema will work better if you
define the @cacheControl directive explicitly.

AS2 did allow you to define the directive yourself (it only adds it if
it isn't there).

So this PR changes apollo-server-core to no longer auto-include the
definitions, and documents how to write them yourself.

(In the future, we may want to change @cacheControl to be a Core
Schema feature, but we're not doing that today.)

(AS2 had a similar issue with the types related to graphql-upload but
that integration is removed.)

Fixes #5181.

[glasser]

FWIW it looks like the insertion was added as part of graphql 14 support in v2.0.6 at a point where changing to require the directives to be defined would be backwards-incompatible. I can't figure out from archaeology why modules, which post-dates v2.0.6, doesn't include the definitions.

[glasser]

On Slack, @abernix and @martijnwalraven signed off on the concept behind this PR; post-merge review of the implementation would be appreciated.

[abernix]

LGTM