Implement remaining fields for certificate_template resource

[aidy]

I only implemented a minimal set of fields[0] as a proof of concept.
We should implement the remaining fields, and allow the defaults to be overridden.

https://developer.venafi.com/tlsprotectcloud/reference/certificateissuingtemplate_create

[0] This is probably true for all resources, but especially for this one

[hawksight]

Eg. make this stuff configurable: https://github.com/jetstack/terraform-provider-tlspc/blob/main/internal/provider/certificate_template_resource.go#L126-L132

[aidy]

fwiw, for those particular fields, I was playing with an interface for this that looked like:

resource "tlspc_certificate_template" "foo" {
    subject_regexes {
        cn = [".*"]
        o   = [".*"]
        ...
    }
}