Supported output credentials for attribute propagation. Each
output credential maps to a "field" in the response. For
example, selecting JWT will propagate all attributes in the IAP
JWT, header in the headers, etc.
Values:
OUTPUT_CREDENTIALS_UNSPECIFIED (0):
An output credential is required.
HEADER (1):
Propagate attributes in the headers with
"x-goog-iap-attr-" prefix.
JWT (2):
Propagate attributes in the JWT of the form:
"additional_claims": { "my_attribute": ["value1", "value2"] }
RCTOKEN (3):
Propagate attributes in the RCToken of the form:
"additional_claims": { "my_attribute": ["value1", "value2"] }