Backport of #2044.

This removes the last dependency on Apollo Server from Apollo Gateway.

Part of apollographql/apollo-server#6057 and
apollographql/apollo-server#6719

The new package `@apollo/server-gateway-interface` (in the apollo-utils
repo for now, though it should move to the apollo-server repo once AS4
is fully released) defines types that are pretty close to compatible
with the AS3 types previously used here, but don't require a dependency
on the entirety of AS3. This new package will be used by AS4 and AS4
will convert its data into the format described by these types.

This change is entirely a build-time change (other than a slight change
to how a enum is referenced). So the worst case scenario if this differs
unintentionally from the original AS3 definitions is that users can
apply a bit of `as any` to fix it.

Note that we've removed some `<TContext>` from types that it turned out
were only ever instantiated with `Record<string, any>` anyway. They are
left in in RemoteGraphQLDataSource because users making their own data
sources can explicitly specify their context type.

The types in `@apollo/server-gateway-interface` are pretty close to the
AS3 types (with different names) but there are some slight differences.
The cache scope enum is replaced with `any`, as enums are not
structurally typed and it is otherwise difficult to type them.
`GatewayInterface` now expects `onSchemaLoadOrUpdate` to exist and
doesn't mention the old `onSchemaChange`.

For now, we leave `apollo-reporting-protobuf` alone, so we don't have a direct
dependency on a prerelease.