Self signed issuer

[RaphaelVogel]

What this PR does / why we need it:
Creating a self-signed certificate was already possible using a CA issuer. Using this approach you need to manually create a self-signed certificate using openssl, create a secret out of it and reference this secret in your CA issuer.

To simplify this manual process a new issuer of type SelfSigned is created, which creates a self-signed certificate.

In addition, two additional features are added:

1. The certificate resource can now define a duration (lifetime of the certificate). This field may be ignored by the issuer (especially Let's encrypt) 👉 introduced via feat: Introduce certificate duration #354
2. Specifying a csr is now possible with issuers of type SelfSigned and ca 👉 introduced via fix creating certificates with a given csr referencing a ca issuer #234

Which issue(s) this PR fixes:
Fixes #183

Special notes for your reviewer:

Release note:

Introduce the new Issuer type `SelfSigned` for creating self-signed certificates.

[gardener-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[gardener-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please ask for approval from martinweindel. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[MartinWeindel]

Thanks for your contribution.
I have some requests for improvement:

• duration should also be handled by CA and ACME issuers
• creating a CA certificate with .spec.csr does not yet work correctly

[marc1404]

/cla

[gardener-prow]

Successfully reached out to cla-assistant.io to initialize recheck of PR #228

[gardener-prow]

@RaphaelVogel: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cert-management-check-renovate-config	32bd917	link	true	/test pull-cert-management-check-renovate-config
pull-cert-management-e2e-kind	b3050b9	link	true	/test pull-cert-management-e2e-kind

Full PR test history. Your PR dashboard. Command help for this repository.
Please help us cut down on flakes by linking this test failure to an open flake report or filing a new flake report if you can't find an existing one. Also see our testing guideline for how to avoid and hunt flakes.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.