autoendpoint: modify FCMCredentials to accept String

[oremj]

Can we modify FCM credentials to accept a string instead of PathBuf? It looks like the yup_oauth2 library only supports a Path in read_service_account_key, but perhaps we could create a temp file out of the auth file string and pass that to the library?

[jrconlin]

Maybe? Kind of the point you're making here. There might be a way to do it using file handoffs, but it's going to be messy because I don't always know if I have write access.

I can try writing to /tmp but I wonder if that might be a security concern.

[pjenvey]

The ServiceAccountKey it returns is just a json deserialization of the creds and it's public, so we should be able to just deserialize directly to it from the env String.

See the read_service_account_key code for what I mean

[oremj]

The ServiceAccountKey it returns is just a json deserialization of the creds and it's public, so we should be able to just deserialize directly to it from the env String.

See the read_service_account_key code for what I mean

This is a good point and likely a good way around the tmp file solution.