Support different formats/templates for secret targets

[knelasevero]

Describe the solution you'd like
Having the ability to define a template to be used to produce a specific type of secret. Like:

Builtin Type	Usage
Opaque	arbitrary user-defined data
kubernetes.io/service-account-token	service account token
kubernetes.io/dockercfg	serialized ~/.dockercfg file
kubernetes.io/dockerconfigjson	serialized ~/.docker/config.json file
kubernetes.io/basic-auth	credentials for basic authentication
kubernetes.io/ssh-auth	credentials for SSH authentication
kubernetes.io/tls	data for a TLS client or server
bootstrap.kubernetes.io/token	bootstrap token data

What is the added value?
More flexibility with the generated secrets. Users can just use what aligns with their use case.

Give us examples of the outcome

This comes from the common CRD proposal as well so it would be defined like this in the ExternalSecret:

  target:
    name: my-secret
    template:
      type: kubernetes.io/TLS
  data:
    tls.crt:
      key: /corp.org/dev/certs/ingress
      property: pubcert
      version: latest
    tls.key:
      key: /corp.org/dev/certs/ingress
      property: privkey
      version: latest

This would generate:

apiVersion: v1
kind: Secret
metadata:
  name: my-secret
type: kubernetes.io/tls
data:
  # the data is abbreviated in this example
  tls.crt: |
    MIIC2DCCAcCgAwIBAgIBATANBgkqh ...
  tls.key: |
    MIIEpgIBAAKCAQEA7yn3bRHQ5FHMQ ...

Observations (Constraints, Context, etc):