Extract ApolloServerPluginUsageReporting from EngineReportingAgent

Remove apollo-engine-reporting package entirely.

There was a function handleLegacyOptions which handled two specific
options (privateHeaders and privateVariables); rewrite it as
legacyOptionsToPluginOptions which turns *all* of the (still exported)
EngineReportingOptions into ApolloServerPluginUsageReportingOptions. This also
includes normalizing maskErrorDetails which used to happen in the ApolloServer
constructor.

Note that if you are using the overrideReportedSchema option to
ApolloServerPluginSchemaReporting, you need to make sure to pass the same option
to ApolloServerPluginUsageReporting. That will ensure that links from traces to
their schemas (when implemented in Studio) work properly. (If this turns out to
be a huge hassle we can incorporate it into the plugin API but I'm not sure if
anyone is actually using this option.)

We no longer warn if you configure usage reporting (explicitly via
ApolloServerPluginUsageReporting or implicitly by setting an API key) with a
schema that appears to be a federated implementing service. (This may be an
explicitly supported mode of operation as managed federation evolves.) Instead,
we *always* log an info line if ApolloServerPluginUsageReporting is operating.

I changed the signature cache to use `lru-cache` directly instead of
InMemoryLRUCache; the main difference is that the APIs are not async
functions (which in InMemoryLRUCache's case always return synchronously
anyway). If we later allow the cache to be customized we can always make it
async again.

package.json

@@ -32,7 +32,6 @@
     "apollo-cache-control": "file:packages/apollo-cache-control",
     "apollo-datasource": "file:packages/apollo-datasource",
     "apollo-datasource-rest": "file:packages/apollo-datasource-rest",
-    "apollo-engine-reporting": "file:packages/apollo-engine-reporting",
     "apollo-federation-integration-testsuite": "file:packages/apollo-federation-integration-testsuite",
     "apollo-reporting-protobuf": "file:packages/apollo-reporting-protobuf",
     "apollo-server": "file:packages/apollo-server",