Self signed issuer

[RaphaelVogel]

What this PR does / why we need it:
Create 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)
2. Specifying a csr is now possible with issuers of type selfSigned and ca

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

[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]

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[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

[MartinWeindel]

Found some nits.

[RaphaelVogel]

/test all