logs should include a format field

[willscott]

caboose will also need to update its logs. we can add a format log field to differentiate between block and car requests

[guanzo]

Let's instead add a request headers field to the log. Just add the Accept header for now, with either application/vnd.ipld.raw or application/vnd.ipld.car as the value. This will keep things extensible with minimal future modifications.

example:

const log = {
  headers: {
    Accept: "application/vnd.ipld.car"
  }
  // ...other fields
}