Add Support for `imagePullSecrets`, cleanup using `helpers.tpl`

Adding support for imagePullSecrets + cleanup using helpers.tpl.

Details

The helm charts currently do not support imagePullSecrets, which is often needed if the enduser wants to provide a private repository for their images. This PR adds support for that
The PR also cleans up labels and selectorLabels to avoid duplication across files, and defines them in _helpers.tpl instead, cleaning up the chart.
Update service.name in ingress.yaml to point to "wiremock.fullname". This avoids issues where the user uses a name other than wiremock as the helm release name during installation. (If something else is used, currently the ingress resource starts pointing to a non-existent resource since the service created has its name set to fullname

[ ] Recommended: Join WireMock Slack to get any help in #help-contributing or a project-specific channel like #wiremock-java
[X] The PR request is well described and justified, including the body and the references
[X] The PR title represents the desired changelog entry
[X] The repository's code style is followed (see the contributing guide)
[X] Test coverage that demonstrates that the change works as expected
[X] For new features, there's necessary documentation in this pull request or in a subsequent PR to wiremock.org