Content trust support

**Description of the issue**:

Add support for signed image , aka content trust.

**Expected behavior**:

Add a key config in the builder to sign the image and wire it in build plugins. For mavennit will use servers typically.

**Steps to reproduce**:

**Environment**:

**`jib-maven-plugin` Configuration**:
```xml
PASTE YOUR pom.xml CONFIGURATION HERE
```

**`jib-gradle-plugin` Configuration**:
```groovy
PASTE YOUR build.gradle CONFIGURATION HERE
```

**Log output**:

**Additional Information**: