Add `metadata` property (and accompanying update) to `assets.json`

This is the contents of an assets.json right now:

```json
{
"AssetsRepo": "Azure/azure-sdk-assets",
"AssetsRepoPrefixPath": "python",
"TagPrefix": "python/keyvault/azure-keyvault-keys",
"Tag": "python/keyvault/azure-keyvault-keys_a5a3c2671c"
}
```

The proposal is to begin populating further metadata under a dictionary here:

```json
{
"AssetsRepo": "Azure/azure-sdk-assets",
"AssetsRepoPrefixPath": "python",
"TagPrefix": "python/keyvault/azure-keyvault-keys",
"Tag": "python/keyvault/azure-keyvault-keys_a5a3c2671c"
"MetaData": {
"commitSHA": "",
"previousTag": "..."
}
}
```

This will be used to power `config diff` as well as the link to the assets.diff for a given assets.json.

This feature must handle non-presence of these attributes if the assets.json hasn't been updated with them. _Every_ operation that would "update" the assets.json will take care of adding the new property though.