Add multiple target params for GaarfBqConfig
Add multiple target params for GaarfBqConfig

When running `gaarf-bq` multiple times with different `--target`
parameters we want to save all them in `config.yaml` file.

* Add changes to `prepare_config` method of `ConfigSaver` -if there's
  an existing `target` in config - add new target(s) to it
* Add `_handle_target_param` static method with either appends or
  extends list depending on what `target` is represented in
  `config.yaml` (as str or list)

Change-Id: Ie88c6e060463497fb6c1c4a48fe1d1b6c6d00223

google/ads-api-report-fetcher,google/ads-api-report-fetcher,google/ads-api-report-fetcher,google/ads-api-report-fetcher