Validate Consul secrets while respecting federation

[t-eckert]

Changes proposed in this PR:

• When installing Consul as a secondary datacenter, the validation which checks for existing Consul secrets will expect a federation secret.
• A release.Release struct was added to encapsulate a Consul release and its configuration. Its use and feature set will be expanded later.
• A helm.Values struct was added, generated from the current Helm values YAML file. This should be generated by the Helm chart and taken as a dependency in the future to avoid getting out of sync.

How I've tested this PR:

• Modified the existing test which checks for Consul secrets to test the behavior which expects an existing federation secret.
• Added tests around the ListConsulSecrets function in the new validation package (which I plan on expanding over time to dedupe validation code.

How I expect reviewers to test this PR:

• Running unit tests
• Use the CLI to deploy a secondary DC following the steps outlined here: https://www.consul.io/docs/k8s/installation/multi-cluster/kubernetes

Checklist:

• Tests added
• CHANGELOG entry added

  HashiCorp engineers only, community PRs should not add a changelog entry.
  Entries should use present tense (e.g. Add support for...)

[jmurret]

Great work. This looks great. Just a few questions but nothing blocking.

[curtbushko]

Looks good!