This was broken because the return value of `initialize()` was a `Promise<...>` but its caller wasn't `await`-ing the value.
This was causing the `gcpIap` provider to fail on the `/request` endpoint because `jwtHeader` was undefined.
The OAuth equivalent keeps `initialize()` synchronous, and all implementations don't _need_ to be `async`, so make them synchronous instead.
I've chosen the changelog to be a `major` bump since this changes the API of a public type.
Signed-off-by: Mitchell Hentges <mhentges@spotify.com>
Mitchell Hentges