Add namespace override

[joshRooz]

Changes proposed in this PR:
Add consul.namespace to helpers, allowing Consul resources to be deployed to a namespace other than .Release.Namespace by setting the global.namespace value.

Derived from #3016 and credit to @0011blindmice.

How I've tested this PR:
Checked out 1.2.2 tag and cherry-picked the PR commits for helm package. Tested in KinD with no global.namespace specified, and with the config shown below.

Helm Values

release_ns=helm   # kube namespace for helm release 
deploy_ns=consul  # kube namespace for consul resources 
helm install consul ./consul-1.2.2.tgz \
  --namespace $release_ns \
  --create-namespace \
  --set global.name=consul \
  --set global.namespace=$deploy_ns \
  --set global.peering.enabled=true \
  --set global.adminPartitions.enabled=true \
  --set global.image=hashicorp/consul-enterprise:1.16.2-ent \
  --set global.datacenter=${deploy_dc:-dc1} \
  --set global.gossipEncryption.autoGenerate=true \
  --set global.tls.enabled=true \
  --set global.enableConsulNamespaces=true \
  --set global.acls.manageSystemACLs=true \
  --set global.enterpriseLicense.secretName=consul-license \
  --set global.enterpriseLicense.secretKey=consul.hclic \
  --set server.replicas=3 \
  --set server.resources.limits.memory=512Mi \
  --set server.exposeService.enabled=false \
  --set connectInject.default=true \
  --set connectInject.apiGateway.managedGatewayClass.serviceType=ClusterIP \
  --set connectInject.cni.enabled=true \
  --set connectInject.cni.logLevel=trace \
  --set connectInject.cni.namespace=kube-system \
  --set connectInject.k8sDenyNamespaces[0]=sync \
  --set meshGateway.enabled=true \
  --set meshGateway.service.type=ClusterIP \
  --set ingressGateways.enabled=true \
  --set terminatingGateways.enabled=true

To verify backwards compatibility, the same values from above were supplied to helm install consul hashicorp/consul --version 1.2.2 .... Subsequently running helm diff upgrade consul ./consul-1.2.2.tgz ..., with the same values, returned no changes.

How I expect reviewers to test this PR:

• Not all componentry was tested/validated. The PR is small change but cascading impact across the whole chart, so edge case validation comes to mind.
• Upgrading and changing global.namespace not in scope, but remains consistent with current expectation that .Release.Namespace does not change after initial installation.

Checklist:

• Tests added
• CHANGELOG entry added

[thisisnotashwin]

Great work here! thanks so much for the contribution.