Cloud Function Service Client load credentials through String

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**Is your feature request related to a problem? Please describe.**
I want to load my GCP credentials for the Cloud Function Service Client thru env variable of json string (that contains the contents of GCP credentials) but I can't find it [here](https://github.com/googleapis/google-cloud-java/blob/0d6459c4c189f572e07640b3789f0cc8cd2a2a45/java-functions/google-cloud-functions/src/main/java/com/google/cloud/functions/v1/CloudFunctionsServiceClient.java#L151). Is it not supported?
```json
{
"type": "external_account",
"audience": "//iam.googleapis.com/projects/blabla/locations/blabla/workloadIdentityPools/...",
"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://someip/latest/meta-data/placement/availability-zone",
"url": "http://someip/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": "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/blablabla"
}
```

**Describe the solution you'd like**
I would like a constructor for this client that uses a json string of the contents of the credentials file.
```java
### JsonNode type
public static final CloudFunctionsServiceClient create(JsonNode credentials) throws IOException {
return create(CloudFunctionsServiceSettings.newBuilder().build());
}

### String type
public static final CloudFunctionsServiceClient create(String jsonCredentials) throws IOException {
return create(CloudFunctionsServiceSettings.newBuilder().build());
}
```

**Describe alternatives you've considered**
Used the file and the credential json file path set as an environment variable, but our security measures dictate that no credentials are to be saved as files.