feat(argo-cd): Allow configuring external redis endpoint from secret

[jdcmarques]

Description:
We are using Argo CD with AWS Elasticache, our Elasticache is provisioned via crossplane alongside the chart.
To be able to correctly point Argo we need to do two installs, or have Elasticache created separately because of the endpoint.
With this change we allow the redis endpoint when using externalRedis to be passed from the secret, it will be up to the user to ensure it is in host:port format

Fixes #3735

Checklist:

• I have bumped the chart version according to versioning
• I have updated the documentation according to documentation
• I have updated the chart changelog with all the changes that come with this pull request according to changelog.
• Any new values are backwards compatible and/or have sensible default.
• I have signed off all my commits as required by DCO.
• I have created a separate pull request for each chart according to pull requests
• My build is green (troubleshooting builds).

[github-actions]

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[mkilchhofer]

Sorry for the late reply. Maybe it would make sense to refactor the externalRedis.existingSecret-feature completely with a structure like this:

externalRedis:

  # ..

  existingSecret:
    name: "asdf"
    serverKey: "xyz"
    usernameKey: "abc"
    passwordKey: "foobar"

Hopefully this would solve other feature requests more easily like this one over there:

• fix(argo-cd): fixed externally managed redis secret marked as optional (#2836) #3849

For sure we'd need to bump the major version of the chart, but that's fully okay for me.

What do you think, @jdcmarques ?