feat: support single-line json as log format

[czenker]

Also introduces a structure to add more log formats should the need arise. Does not change the default of logging prettified json.

Description

Log Shippers are usually less complex and less error-prone when they only have to handle single-line logs. This PR adds the ability to change the log format to single-line json, but does not change the default multi-line json.

Checklist

• [x] PR is rebased to/aimed at branch develop
• [x] PR follows Contributing Guide
• [x] Added tests (if necessary)
• [ ] Extended README/Documentation (if necessary)
• [ ] Adjusted versions of image and Helm chart in Chart.yaml (if necessary)