Decode key used for image decryption

[chendave]

The key should be base64 encoded if it's from a kbs repository, KMS or file system instead of plain text.

This align with other KBC module, suh as offline_fs_kbc.

guest-components/attestation-agent/kbc/src/offline_fs_kbc/common.rs

Lines 17 to 26 in dec7f10

	let encoded_keys: HashMap<String, String> =
	serde_json::from_str(&keys_json).map_err(|_| anyhow!("Failed to parse keys JSON file"))?;
	encoded_keys
	.iter()
	.map(
	|(k, v)| match base64::engine::general_purpose::STANDARD.decode(v) {
	Ok(key) => Ok((k.clone(), key)),
	Err(_) => Err(anyhow!("Failed to decode key")),
	},
	)

[chendave]

talked with @Xynnn007 , this is expected, the key is not base64 encoded.