Fix register a key using secret creationTimestamp instead of certificate validity timestamp

[sybernatus]

Description of the change

Register a key and order it based on its secret creation timestamp instead of its cert starting validity (NotBefore attribute)

Benefits

In case we are bringing our own certificate, if the certificate is created before sealed secret installation but applied after the installation, still the latest certificate used will be the certificate installed by sealed secret during its initialization instead of the latest secret certificate created.

Possible drawbacks

Applicable issues

• fixes # Bring-your-own-certificate - controller does not fetch latest key imported #1639

Additional information

[alvneiayu]

hi @sybernatus

First of all, thanks a lot for your PR.

This PR will change the way to order the certificates for all the people with Sealed Secrets deployed. This impact is really high if we include it as default.

I suggest to include it with a flag to configure what is the way that the users want to order the certificates. If we see in the future that everybody is using the flag, we can deprecated it and set it by default but I think that we need to be conservative here.
I though something like --order-by-creation-timestamp.

For this case, we need to include a test and documentation.

Thanks a lot

Álvaro