fix template type error

[sbaier1]

What this PR does / why we need it:

The template field in ExternalSecrets objects is currently broken, as the K8s client will attempt to decode the object as base64 first, which fails. The correct type has to be used in the internal datastructure.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #83

Other notes for your reviewer:
Not sure if this is the best approach to fix the problem but form my research into the client library this is the specific type intended for this type of usage.

It may also be possible to directly use a Secret type object in the CRD and internal structure and stop having to sacrifice CRD schema structure registration, but that would collide with the intent of other pending issues like #22

Release note:

Fixed an issue that lead to sync errors when trying to use the template field in ExternalSecret objects.