Use data.external to execute a CLI script:
credentials=$(aws --profile dev iam list-service-specific-credentials \
  --user-name jenkins --service-name codecommit.amazonaws.com --query 'ServiceSpecificCredentials[0]')
if [[ $credentials == "null" ]]; then
  credentials=$(aws --profile dev iam create-service-specific-credential --user-name jenkins \
  --service-name codecommit.amazonaws.com --query ServiceSpecificCredential)
fi
echo "$credentials"
Then the terraform:
data "external" "jenkins" {
  count   = "${local.is_pcs_dev}"
  program = ["${path.root}/jenkins.sh"]
}
resource "aws_ssm_parameter" "jenkins_cc_id" {
  name      = "${local.jenkins}/codecommit_https_user"
  value     = "${lookup(data.external.jenkins.result, "ServiceUserName", "")}"
}
resource "aws_ssm_parameter" "jenkins_cc_p" {
  name      = "${local.jenkins}/codecommit_https_pass"
  value     = "${lookup(data.external.jenkins.result, "ServicePassword", "")}"
}