Unify expiry buffer accross electron auth clients

added ability to set expiryBuffer for ElectronRendererAuthorization, defaulting to 10 min
added defaultExpiryBufferInSeconds to be reused in both electron auth clients
added ElectronRendererAuthorization._tokenRequest to be returned instead of Promise.reject() to short-circuit any recursive use of ElectronRendererAuthorization.getAccessToken

Fixes #232

iTwin / auth-clients