[Asset Sync] Add an `assets.json` field that links directly to a package's assets

Currently, the "golden path" for a user to find their library's assets is to:

1. Open their `assets.json` file
2. Manually go to the repository denoted by the `AssetsRepo`
3. Manually select the repo tag denoted by the `Tag`

Instead, we can provide a direct link to the repo tag and eliminate steps 2 and 3 by using currently-available information. For example, the [`assets.json` file for Python's `azure-keyvault-secrets`](https://github.com/Azure/azure-sdk-for-python/blob/75e16c0dc856d4a23f6e080fec8302fd52246da9/sdk/keyvault/azure-keyvault-secrets/assets.json) currently looks like:
```json
{
"AssetsRepo": "Azure/azure-sdk-assets",
"AssetsRepoPrefixPath": "python",
"TagPrefix": "python/keyvault/azure-keyvault-secrets",
"Tag": "python/keyvault/azure-keyvault-secrets_fc5951fbe4"
}
```

By using the `AssetsRepo` and `Tag` fields, we can generate a direct link to the tag with
```
https://github.com/{AssetsRepo}/tree/{Tag}
```
and in the example above's case, add an extra field:
```json
"AssetsLocation": "https://github.com/Azure/azure-sdk-assets/tree/python/keyvault/azure-keyvault-secrets_fc5951fbe4"
```