Share Zoom access token with Datastore

[harveysanders]

This
https://cloud.google.com/appengine/docs/legacy/standard/go111/datastore/api-overview

Is your feature request related to a problem? Please describe.
Currently, we're fetching the Zoom access token on every request. We used to request a new token if the current token was expired, but since Zoom only allows one valid access token at a time, we ran into a race condition when another Function instance would request a new token.
Ex:
Instance A spins up and requests a token.
Soon after, instance B spins up and requests a new token, invalidating instance A's token
Then if A served another request, it would attempt to use the invalid previous token, since it has not yet expired.

Describe the solution you'd like
We could share one token using Datastore.

Pseudocode

Function

get token from Datastore
if token expired?
  request new token
  store token and expiresIn values in Datastore
  return token

return valid token  

Describe alternatives you've considered
Requesting a new token every invocation seems to work well enough now, but I think will fail with concurrent spikes of requests.

Additional context

• Original version:

  service-signups/zoom.go

  Lines 86 to 90 in f081812

  	if !z.isAuthenticated() {
  	if err := z.authenticate(ctx); err != nil {
  	return fmt.Errorf("authenticate: %w", err)
  	}
  	}

• Attempted fix to share token in Function context: Use global zoomCreds variable #41
• New token on every request: Fix/zoom auth #42

[harveysanders]

Apparently we're not the only one dealing with the issue: https://devforum.zoom.us/t/frequently-receiving-invalid-request-token-revocation-from-zoom/44220/16