SecretTemplate supports ytt templating

**Describe the problem/challenge you have**

https://github.com/vmware-tanzu/carvel-secretgen-controller/pull/66 Introduced the SecretTemplate API. This allows you to define arbitrary input resources that are used to generate a new Kubernetes Secret based on templating.

For a first pass, we just supported a simple jsonpath based template. However for more dynamic cases we should support a ytt templating. This would allow the entire generated secret to be templated.

**Describe the solution you'd like**
We should introduce an optional `ytt` block as an alternative to `template` that loads input resources as data.values. So this might look like:
```yaml
---
apiVersion: secretgen.k14s.io/v1alpha1
kind: SecretTemplate
metadata:
name: generated-secret
spec:
serviceAccountName: my-resource-reader
inputResources:
- name: rds
ref:
apiVersion: rds.services.k8s.aws/v1alpha1
kind: DBInstance
name: my-rds-instance
- name: creds
ref:
apiVersion: v1
kind: Secret
name: {.rds.masterPassword.name}
namespace: rds-services
ytt: |
#@ load("@ytt:data", "data")

# Store the input resource as data values with the key being
# the specified reference name.
#@ rds = data.values.rds
#@ creds = data.values.creds
#@ endpoint = rds.status.endpoint
metadata:
name: db-instance-secret
stringData:
type: postgresql
database: #@ rds.spec.dbName
port: #@ endpoint.port
host: #@ endpoint.address
# Example of defaulting.
username: #@ rds.spec.masterUsername if rds.spec.masterUsername != "" else "admin"

data:
# Example of dynamic key loading.
password: #@ creds.data[rds.masterUserPassword.get("key")]
```

**Anything else you would like to add:**
n/a

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