Plugins: no server; interface contravariant; no factory functions

[glasser]

Previously on version-4, we had made ApolloServerPlugin<TContext>
"invariant" in TContext via the new in out annotation. That's
because we had added a server field to GraphQLRequestContext and
GraphQLServerContext.

The issue here is that because you can invoke
server.executeOperation(op, contextValue), then a
ApolloServerPlugin<BaseContext> was not assignable to
ApolloServerPlugin<SomeSpecificContext>, because the former was able
to call requestContext.executeOperation(op, {}), and a server whose
context has additional required fields could not load this plugin. This
essentially meant that you couldn't have a single object (not created by
a generic function or generic class) that was a plugin that works with
any server, even if it doesn't actually care about context values at
all.

When trying to dogfood AS4 it became clear that this was annoying. So
let's fix it by removing server from those objects and making
ApolloServerPlugin "contravariant" instead. This means that plugins
can only receive contextValue, not send it. And so
ApolloServerPlugin<BaseContext> (a plugin that doesn't bother to read
any fields from contextValue) can be assigned to
ApolloServerPlugin<SpecificContext> (a plugin that is permitted to
read specific fields on contextValue, though it may choose not to).

After removing server, restore logger (now readonly) and cache
to GraphQLRequestContext and GraphQLServerContext (cache is
actually new to GraphQLServerContext in AS4). They had previously been
removed for redundancy.

This un-does the fix to #6264. If your plugin needs access to the
server object, you can just pass it to your plugin's
constructor/factory yourself. This wasn't an option in AS3 because there
was no server.addPlugin method... or actually, you could do it with
the plugin factory feature.

Note that we do leave ApolloServer<TContext> as invariant in
TContext, for the reasons described in a comment above the class.

While we're at it, remove the unnecessary ability to specify plugins as
factory functions. As far as we know, the main use case for this feature
is to refer to the ApolloServer itself in the expression creating the
plugin, but the new addPlugin method lets you do the same thing.
Reducing the number of ways to specify constructor options helps keep
type errors simpler.

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[glasser]

Going to merge this without full review so that I can make another alpha and move forward, but happy to make big changes or even revert after feedback.