feat: Add templating functionality to extraContainers

[otherguy]

Thanks for submitting a PR! Please check the boxes below:

• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have bumped the version number in /charts/flagsmith/Chart.yaml in the section version or I'm merging to a
  release branch

Changes

Allow templating in extraContainers, closing #304.

Adjust template logic for extraContainers to support both string and non-string types. Strings are evaluated as templates, similar to extraObjects.

How did you test this code?

Manually.

$ helm template flagsmith --dry-run --debug charts/flagsmith/ -f flagsmith.values.yaml

 # flagsmith.values.yaml

 # ...
 extraContainers: |
    - name: cloud-sql-proxy
      image: {{ $.Values.sqlProxySidecar.image.repository }}:{{ $.Values.sqlProxySidecar.image.tag }}
      imagePullPolicy: IfNotPresent
      command:
        # Start Cloud SQL Proxy
        - /cloud-sql-proxy

        # Add structured logs as JSON
        - --structured-logs
 # ...

[khvn26]

This is a great addition — thanks a lot!

[matthewelwell]

@khvn26 does this also need documentation updates?

[khvn26]

@matthewelwell Both extraContainers and extraVolumes need to be added to docs, as well as extraObjects.