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.

Start in java-functions/google-cloud-functions/src/main/java/com/google/cloud/functions/v1/CloudFunctionsServiceClient.java at the credential-loading entry point linked in the issue, then compare it with CloudFunctionsServiceSettings. Check how a JSON credential string could be passed without a file; done means the Cloud Functions client can be created from the requested credential contents and the behavior is covered by appropriate tests.

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