Remove GetIssuerTypeIdentifier from Issuer API

[inteon]

It is now moved to the controller initialization.

Instead having to implement GetIssuerTypeIdentifier for each issuer type, we now just ask to provide the GroupResource when intialising the controller. This makes the API more intuitive.

Later (future PRs), we could use kubernetes discovery to discover the GroupResource based on the installed CRDs.

[terinjokes]

I don't have the right commit bits to move this along, but I'm a fan of simplifying the interface. Would it be feasible to remove GetStatus as well, to not need anything other the functions from client.Object to be implemented by the issuer types?

[cert-manager-prow]

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[cert-manager-prow]

PR needs rebase.

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