Cert-manager: Allow selecting the certificate format to be used

[matthew-muscat]

Is your feature request related to a problem? Please describe.
Currently, certificates keys created by cert-manager are created using the PKCS#1 format, however, in some applications, this format may not be a supported format

Specifically, this is a feature that would enable us to better handle self-signed certificates that we generate using cert-manager for the "Logstash" application, which only currently supports the PCKS#8 format.

Describe the solution you'd like
Similar to keyAlgorithm, allow users to specify a format that the certificate should be made available in.

Depending on preferences here
(1) The format could determine the overall format used for the certificate tls.key and tls.crt values (this is what i was thinking).
(2) Alternatively the format could enable the secret to include multiple files in the formats specified by the user (ie: tls-pkcs8.key, tls-pkcs8.crt)

Describe alternatives you've considered
Currently, we've worked around this by having an initContainer run an openSSL command to convert the contents of the certificate secrets into the format required by the application — however, this has downsides with handling certificate rotation within the pod and introduces external dependencies to get the pod and certificate in place.

/kind feature

[Feder1co5oave]

Note: it would suffice to use MarshalPKCS8PrivateKey instead of MarshalPKCS1PrivateKey here => https://github.com/jetstack/cert-manager/blob/3b9aae318f2bb5bff862b6ac048125651274278b/pkg/util/pki/generate.go#L113-L122

[retest-bot]

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[mohag]

Other formats that might be useful: PFX / PKCF#12 (.NET Core, several things on Windows). Separate chain (Older Apache versions, RabbitMQ), Java keytool keystore (Likely ugly, but useful...) (with single cert, key, chain and CA (if available)), NSS (certutil managed) databases... (e.g. for Apache with mod_nss)