Make the plugin (and gateway) API almost entirely async

[glasser]

Before this PR, some plugin methods were sync, some were async, and some
returned ValueOrPromise (ie, could be sync or async). This led to a lack
of functionality for the never-async methods, confusing inconsistency,
and generally vagueness as to what types things are.

It's 2021: making a function async is as simple as adding the word
async and maybe wrapping a return type in Promise<>. This PR
simplifies matters by making almost every plugin and gateway method
always async.

The one exception is willResolveField, which is called far more than
any other plugin method. We already know that schema instrumentation has
unfortunate performance overhead so adding to it by adding more Promises
to every field doesn't seem like a good idea for now. We reserve the
write to change willResolveField to a PromiseOrValue sometimes-async
method later.

(Note that in practice, we usually either await or call Promise.all
on the return values from plugins rather than directly calling .then,
so if you're not using TypeScript you can probably get away with writing
sync methods; and if you are using TypeScript the compiler will quickly
show you where you're missing your asyncs.)

Specifically, this PR:

• Makes the following formerly-ValueOrPromise methods into async
  methods: serverWillStart, serverWillStop, didResolveSource,
  didResolveOperation, didEncounterErrors, responseForOperation,
  willSendResponse
• Makes the following formerly-sync methods into async methods:
  requestDidStart, renderLandingPage, parsingDidStart and its stop
  hook, validationDidStart and its stop hook, executionDidStart,
  executionDidEnd
• executionWillStart can no longer longer return an end hook as a
  function; executionWillEnd must be returned in an object (which was
  one of two options before)
• Changes the methods on Dispatcher so that there's only one "sync"
  method (for willResolveField) and the other methods aren't explicitly
  named with Async
• Simplifies the GraphQLService interface (used for @apollo/gateway)
  to require the stop method to exist, to require executor to be
  async, and to note that we always pass apollo to load. (All recent
  versions of @apollo/gateway satisfy this.) Also require the
  GraphQLExecutor function type to be async.

This also avoids the use of ValueOrPromise<void>, a somewhat confusing
type that means "either a Promise or a return value we shouldn't look
at" though we do still have Promise<X|void> types.

Note that we allow options and context functions to still have
ValueOrPromise semantics.

Fixes #4103. Fixes #4999. Incorporates work by @lucasconstantino from
#4050 and #4051.

[glasser]

Docs need to be updated, by which I mean I need to search for every single plugin method name and make sure all examples show them as async.