Add secrets support for adhoc validations.

Some validations require to upload reports to different places. This CL
adds support to download, decrypt and use secret values in validations.

Change-Id: Iae49a40c32564138732f6ed567eaa07efc4c629f
Bug: https://github.com/flutter/flutter/issues/64796
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/6160
Commit-Queue: Godofredo Contreras <godofredoc@google.com>
Reviewed-by: Tong Wu <wutong@google.com>