Add Gradle task to upload symbols and mapping

[kstenerud]

Adds upload tasks that can be used by customers in their Gradle scripts.

These tasks are available when customers add the capture plugin to their plugins definition in their app's build.gradle:

plugins {
    ...
    id 'io.bitdrift.capture-plugin' version '0.16.7'
}

$ ./gradlew tasks
...
Upload tasks
------------
bdUpload - Upload all symbol and mapping files to Bitdrift
bdUploadMapping - Upload mapping to Bitdrift
bdUploadSymbols - Upload symbols to Bitdrift
...

Note: API_KEY must be set in the shell environment or else the tasks will complain:

* What went wrong:
Execution failed for task ':app:bdUploadMapping'.
> Environment variable API_KEY must be set to your Bitdrift API key before running this task