POST /v1/projects/-/serviceAccounts/fake@example.com:signBlob HTTP/1.1
Content-Length: 71
user-agent: ericnorris/gcp-auth-contrib unit test
Host: iamcredentials.googleapis.com
Accept: application/json
Authorization: Bearer a-token-1
Content-Type: application/json

{"payload":"c2lnbmFibGUtc3RyaW5n","delegates":["delegate@example.com"]}