Add a mechanism to prevent concurrent token refreshes

[lilpit]

When multiple different firebase services require a token refresh at the same time, multiple token refresh can be triggered at the same time inside the SecureTokenService.

This PR makes sure only one token refresh is happening at the same time

Discussion

• This is addressing Multiple concurrent token refresh calls on app launch #15474

Testing

• Added some unit tests to test concurrent reload behaviors
• Tested this branch in our sample project where we reproduce the issue -> the issue is gone
  Testing scenario:
• Open the sample app
• Wait for 1 hour for token expirtation
• Re open the sample app -> only one token refresh is performed instead of multiple one

API Changes

• No API changes

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[ncooke3]

Thanks for the PR!

[ncooke3]

@lilpit, thanks again for the fix! Question, in the sample project, was there a purpose to Remote Config for reproducing the original issue? My understanding is that the flow was: sign in, let token expire, re-open app to kick off token refresh and manually trigger the two document refreshes which will also kick spawn token refreshes (3 in total). What role did the remote config fetch play?

https://github.com/thibaudse/firebase-concurrency-issue-mcve/blob/main/firebase-concurrency-issue-mcve/Trigger.swift