feature: letsencrypt http.Handler

[szuecs]

feature: letsencrypt http.Handler integration via autocert with different storage/cache systems to be used by different deployments

[zalando-robot]

Deployment Checklist

This change falls under the deployment policy.

💁 Since Nov 10th, we are in the RED deployment zone. This means all changes released to production must adhere to the following requirements:

• Detailed release notes are provided in this PR’s description.
• Thorough load-testing has been performed, and is documented in the description/comment.
• You can enable/disable the change via feature toggles, and have confirmed these toggles work as expected.
• Technical review: A Principal Engineer, Engineering Manager or Head of Engineering have green-lit your changes, and the reviewer is named in the description/comments.
• Application Owner (Director+) approval is given about the PR, and the approver is named in the description/comments.

👉 Regardless of which boxes you click in this comment, merge/deployment will not be blocked.
Reports about deployment policy adherence will be circulated daily.