GitHub build pipelines

[Radiokot]

Purpose

Added pipelines for internal testing and release builds, using secrets from the GitHub environment.
https://linear.app/concordium/issue/WAL2-71/cryptox-android-create-build-workflow-on-github

Changes

• Add internal testing pipeline that runs for -qa and -rc tags. It builds signed APKs for 3 networks and publishes them for internal testing in Firebase;
• Add release build pipeline that runs for X.Y.Z tags. It builds signed APKs and bundles for Mainnet and Testnet, for Google Play upload and standalone release;
• All the signing keys are merged into signing.jks. Sensitive keys are protected with passwords set in the GitHub environment.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.

[lassemand]

Left a few comments, which you can take or leave.

Good job, looks good to me