Support external_account credential file

I'm trying to integrate grafana in aws and gcp cloud logging using WIF (https://cloud.google.com/iam/docs/workload-identity-federation-with-other-clouds).
As i'm trying to integrate multiple gcp projects in different grafana datasources, I had to upload credential file to plugin config, but it raised error with message `Missing keys: private_key, token_uri, client_email, project_id`.

Please support external_account credential files

```json
{
"universe_domain": "googleapis.com",
"type": "external_account",
"audience": "//iam.googleapis.com/...",
"subject_token_type": "urn:ietf:params:aws:token-type:aws4_request",
"token_url": "https://sts.googleapis.com/v1/token",
"credential_source": {
"environment_id": "aws1",
"region_url": "http://169.254.169.254/latest/meta-data/placement/availability-zone",
"url": "http://169.254.169.254/latest/meta-data/iam/security-credentials",
"regional_cred_verification_url": "https://sts.{region}.amazonaws.com?Action=GetCallerIdentity&Version=2011-06-15"
},
"service_account_impersonation_url": "..."
}
```