Add `soft` field to key descriptions in `keys-mapping.json`

[n-dusan]

Add a soft: true field to key descriptions in keys-mapping.json to distinguish software keys from hardware keys (YubiKeys).

Example:

"example-key": {
    "public": "-----BEGIN PUBLIC KEY-----\n...",
    "keyid": "add12360f",
    "scheme": "rsa-pkcs1v15-sha256",
    "soft": true
}