Stackdriver plugin: Support gzip compression

**Is your feature request related to a problem? Please describe.**
We run fluent-bit in a mobile environment using cellular connectivity, which we pay for. We'd like to reduce the bandwidth consumption.

**Describe the solution you'd like**
We use the Stackdriver plugin to send our logs to Google Cloud Logging. We'd like to compress these HTTP requests.

**Describe alternatives you've considered**
N/A

**Additional context**
It appears from the code that the stackdriver plugin does not support the compression directive.
https://github.com/fluent/fluent-bit/blob/780b36c45f715beb5d7620e67983ef43f402fcec/plugins/out_stackdriver/stackdriver.c#L2424-L2442

In contrast, the HTTP plugin seems to support it.
https://github.com/fluent/fluent-bit/blob/4a2b0febd3eaef18ac04456fcb6ad1f15727a826/plugins/out_http/http.c#L203-L206