Use keystore.properties file for signing bundles

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

I have a step in my CI build which uses the command `build-apks`. In my case these .apk files need to be signed. For this project, we have a file `keystore.properties` in a format like [this](https://developer.android.com/studio/build/gradle-tips#remove-private-signing-information-from-your-project).

With this approach, it is difficult to pass these values to bundletool.

**Describe the solution you'd like**

I would like to pass a single flag like `--keystore-properties=keystore.properties`, to set all of the signing parameters at once.

**Describe alternatives you've considered**
I made a gradle task which extracts the value for each field from `keystore.properties`, and saves each property them into a new file. These files are then copied to variables in fastlane, so that they can be passed back to bundletool