IdentityPoolCredential does not work for text formatted credential source

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subject_token_field_name is only valid for CredentialFormatType.JSON, but is also run on CredentialFormatType.TEXT which cause it to throw the exception:

google-auth-library-java/oauth2_http/java/com/google/auth/oauth2/IdentityPoolCredentials.java

Lines 139 to 146 in 5688837

	credentialFormatType =
	type.equals("text") ? CredentialFormatType.TEXT : CredentialFormatType.JSON;
	if (!formatMap.containsKey("subject_token_field_name")) {
	throw new IllegalArgumentException(
	"When specifying a JSON credential type, the subject_token_field_name must be set.");
	}
	subjectTokenFieldName = formatMap.get("subject_token_field_name");

In the Python SDK, this line is if-checked to run only with JSON:

https://github.com/googleapis/google-auth-library-python/blob/3c3fbf40b07e090f2be7fac5b304dbf438b5cd6c/google/auth/identity_pool.py#L158-L165

Stack trace

java.lang.IllegalArgumentException","message":"When specifying a JSON credential type, the subject_token_field_name must be set.","stacktrace":"java.lang.IllegalArgumentException: When specifying a JSON credential type, the subject_token_field_name must be set.\n\tat com.google.auth.oauth2.IdentityPoolCredentials$IdentityPoolCredentialSource.<init>(IdentityPoolCredentials.java:144)\n\tat com.google.auth.oauth2.ExternalAccountCredentials.fromJson(ExternalAccountCredentials.java:319)\n\tat com.google.auth.oauth2.GoogleCredentials.fromStream(GoogleCredentials.java:170)\n\tat com.google.auth.oauth2.DefaultCredentialsProvider.getDefaultCredentialsUnsynchronized(DefaultCredentialsProvider.java:159)\n\tat com.google.auth.oauth2.DefaultCredentialsProvider.getDefaultCredentials(DefaultCredentialsProvider.java