support http plain text response body

[yiran0427]

Checklist

• I've searched the issue queue to verify this is not a duplicate feature request.
• I've pasted the output of kargo version, if applicable. v1.7.5
• [] I've pasted logs, if applicable.

Proposed Feature

the http promotion step should be able to handle response body that is plain text.

Motivation

use case: i need to curl http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/identity for a auth token, and the response body here is only a string token.

current behaviour:
step configuration

outputs:
- fromExpression: response.body
  name: token

output:

{
 "token": {}
}

i have a PR opened #5106