Support custom http headers in auth scope

**Environment**:

- *Jib version:* 3.4.0
- *Build tool:* Gradle
- *OS:* -

**Description of the issue**:

As a user, I'd like to specify additional headers for a http client that is used by jib. In my case, it's required to properly access the private docker registry, where the additional access token is utilized by a security layer. It's also not something specific to my setup, any user using tunnel-like services (e.g. Cloudflare One) will run into this problem.

**Expected behavior**:

Headers defined in `to`/`from` configuration scope should be attached to the request. Something like this would be good enough to be fair:

```kotlin
jib {
to {
// [...]
auth {
addHeader("XXX-Client-Id", "abc")
addHeaeer("XXX-Access-Key", "cdf")
}
```