Support Backup Outputs

**Is your feature request related to a problem? Please describe.**

**Problem**: Something goes wrong and Fluent Bit is unable to send to your log destination. May be the destination goes down, or some network or other config prevents Fluent Bit from reaching it.

**Proposed Solution**: Specify a second output plugin as backup for another output. You'd be able to configure a timeout/threshold for when Fluent Bit switches to the backup output. (May be number of failed retries, number of errors, or something like the time since the first unsuccessful send).

Compared to other existing feature requests and open issues, I do not think this is very high priority. I'm interested in seeing if anyone else thinks this would be useful. Plus one if you agree and comment if you want to discuss how this would work!