feat: Allow using templates in indexFormat for OpenSearch sink

[jarnfast]

This PR adds the capability to use templates when constructing the indexFormat for OpenSearch sinks.

The change is backwards compatible with the existing time formatting directives - this is ensured by the added tests.

Given an example configuration:

receivers:
  - name: "by-namespace"
    opensearch:
      hosts:
        - http://localhost:9200
      indexFormat: "kube-events-{{ .InvolvedObject.Namespace }}-{2006-01-02}"

An event originating from kube-system will end up in a index named kube-events-kube-system-<go formatted date>.

Closes #155