feat(vector): add support for string templating in extraObjects

[lasermoth]

Description

Adds optional string templating to extraObjects.

This allows more flexibility with extraObjects.

Example Use Case

Conditionally enabling custom resources like Keda ScaledObject if autoscaling is set to external.

Testing

# values-extra-objects.yaml

custom:
  enabled: true
  config: "my custom config"

extraObjects:
  - apiVersion: v1
    kind: ConfigMap
    metadata:
      name: foo
      namespace: "{{ .Release.Namespace }}"
    data:
      type: "yaml"
  - |
    {{- if $.Values.custom.enabled }}
    apiVersion: v1
    kind: ConfigMap
    metadata:
      name: bar
      namespace: "{{ .Release.Namespace }}"
    data:
      type: "string"
      config: "{{ $.Values.custom.config }}"
    {{- end }}

helm template vector . -f values-extra-objects.yaml | yq 'select(.kind == "ConfigMap" and (.metadata.name == "foo" or .metadata.name == "bar"))'

# Source: vector/templates/extra-manifests.yaml
apiVersion: v1
data:
  type: yaml
kind: ConfigMap
metadata:
  name: foo
  namespace: 'default'
---
# Source: vector/templates/extra-manifests.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: bar
  namespace: "default"
data:
  type: "string"
  config: "my custom string"