Backport of [NET-11567] Update Helm Chart to allow snapshotagent to support extraVolumes and extraEnvionmentVars into release/1.6.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #4471 to be assessed for backporting due to the inclusion of the label backport/1.6.x.

The below text is copied from the body of the original PR.

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Changes proposed in this PR

• Add support for server.snapshotAgent.extraEnvironment and server.snapshotAgent.extraVolume in the helm chart to allow for configuring google application credentials

How I've tested this PR

• Running locally against a kind cluster with the snapshot agent configured to point at a google bucket

Using the following config

Google application credentials loaded into cluster with the following secret name/secret key

   name: snapshot-agent-google-app-creds
   key: creds.json

configSecret

kubectl create secret generic snapshot-config --from-file=snapshot-agent=<path-to-config-below>/snapshotagentconfig.yaml

{
  "snapshot_agent": {
    "http_addr": "127.0.0.1:8500",
    "token": "",
    "datacenter": "",
    "ca_file": "",
    "ca_path": "",
    "cert_file": "",
    "key_file": "",
    "license_path": "",
    "tls_server_name": "",
    "login": {
      "auth_method": "",
      "bearer_token": "",
      "bearer_token_file": "",
      "meta": {}
    },
    "log": {
      "level": "INFO",
      "enable_syslog": false,
      "syslog_facility": "LOCAL0"
    },
    "snapshot": {
      "interval": "5m",
      "retain": 30,
      "stale": false,
      "service": "consul-snapshot",
      "deregister_after": "76h",
      "lock_key": "consul-snapshot/lock",
      "max_failures": 3,
      "local_scratch_path": "/home/consul"
    },
    "backup_destinations": {
      "google_storage": [
        {
          "bucket": "snapshot-test-bucket-2-4-2025"
        }
      ]
    }
  }
}

config.yaml

---
global:
  image: 'hashicorp/consul-enterprise:1.20.0-ent'
  enterpriseLicense:
    secretName: 'consul-ent-license'
    secretKey: 'key'

server:
  snapshotAgent:
    enabled: true
    interval: "30s"
    fakevalue: false
    configSecret:
      secretName: snapshot-config
      secretKey: snapshot-agent
    extraEnvironmentVars:
      GOOGLE_APPLICATION_CREDENTIALS: /consul/userconfig/snapshot-agent-google-app-creds/creds.json
    extraVolumes:
      - type: secret
        name: snapshot-agent-google-app-creds

How I expect reviewers to test this PR

• Read and verify screenshots

Checklist

• Tests added
• CHANGELOG entry added

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Overview of commits

• f18107e - 0faeb81