feat: adding namespace override

[brito-rafa]

Adding parameter namespaceOverride as requests on #161.

When using --set namespaceOverride=<namespace>, it is required to pre-create the namespace.

Improved the README as well.

Log of the test:

# create the namespace
% k create ns override

% helm install opencost-override ./charts/opencost --set namespaceOverride=override
NAME: opencost-override
LAST DEPLOYED: Thu Jan 11 14:49:52 2024
NAMESPACE: default  <-----------
STATUS: deployed
REVISION: 1
TEST SUITE: None

% helm install opencost-vanilla ./charts/opencost --namespace=vanilla --create-namespace 
NAME: opencost-vanilla
LAST DEPLOYED: Thu Jan 11 14:50:14 2024
NAMESPACE: vanilla
STATUS: deployed
REVISION: 1
TEST SUITE: None

% k get pod -n override -w
NAME                                 READY   STATUS    RESTARTS   AGE
opencost-override-844bd7498f-w5szq   1/2     Running   0          66s
opencost-override-844bd7498f-w5szq   2/2     Running   0          2m6s   <------ running

% k get pod -n vanilla    
NAME                                READY   STATUS    RESTARTS   AGE
opencost-vanilla-78cb5bfbc9-5jg4j   2/2     Running   0          2m3s