Demos: Allow to specify different source/target key slot number in the halo-backup demo

[icedevml]

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Description

Checklist

Changes to the drivers

• (PR Author) The affected drivers were manually tested

Changes to CLI

• (PR Author) The change was manually tested with the CLI
• (PR Author) The affected CLI features are working with the standalone binary (at least one platform)
• (Checked by maintainer) The CLI test procedure was run by the project's maintainer

Changes to web library

• (PR Author) The change was manually tested with the web library included within a classic HTML application (flat libhalo.js)
• (PR Author) The change was manually tested with the web library included within an app based on frontend framework (React.js or similar based on webpack)
• (Checked by maintainer) The web test suite was run by the project's maintainer

Changes to nfc-manager driver

• (PR Author) The change was manually tested in React Native app
• (Checked by maintainer) The test suite was run through the test React Native project