Convenient way to persistently set cilium namespace

[rptaylor]

Proposal / RFE

Is your feature request related to a problem?

It is inconvenient and distracting to have to remember to add -n cilium when typing every command, especially when copy/pasting from documentation for example.

Describe the solution you'd like
There should be an easy way to persistently configure the namespace of the cilium installation in your environment. For example an env var like CILIUM_NAMESPACE or a ~/.cilium/config file or something. That way you can just set it once and not worry about having to type it every time.

[rptaylor]

Continuation of #2067

[rptaylor]

$ cilium status
    /¯¯\
 /¯¯\__/¯¯\    Cilium:             1 errors
 \__/¯¯\__/    Operator:           disabled
 /¯¯\__/¯¯\    Envoy DaemonSet:    disabled (using embedded mode)
 \__/¯¯\__/    Hubble Relay:       disabled
    \__/       ClusterMesh:        disabled

Containers:            cilium             
                       cilium-operator    
Cluster Pods:          25/25 managed by Cilium
Helm chart version:    
Errors:                cilium    cilium    daemonsets.apps "cilium" not found
status check failed: [daemonsets.apps "cilium" not found, unable to retrieve ConfigMap "cilium-config": configmaps "cilium-config" not found]

$ cilium -n cilium status
    /¯¯\
 /¯¯\__/¯¯\    Cilium:             OK
 \__/¯¯\__/    Operator:           OK
 /¯¯\__/¯¯\    Envoy DaemonSet:    OK
 \__/¯¯\__/    Hubble Relay:       disabled
    \__/       ClusterMesh:        disabled

DaemonSet              cilium             Desired: 6, Ready: 6/6, Available: 6/6
DaemonSet              cilium-envoy       Desired: 6, Ready: 6/6, Available: 6/6
Deployment             cilium-operator    Desired: 2, Ready: 2/2, Available: 2/2
Containers:            cilium             Running: 6
                       cilium-envoy       Running: 6
                       cilium-operator    Running: 2
Cluster Pods:          25/25 managed by Cilium
Helm chart version:    1.16.6
Image versions         cilium             quay.io/cilium/cilium:v1.16.6@sha256:1e0896b1c4c188b4812c7e0bed7ec3f5631388ca88325c1391a0ef9172c448da: 6
                       cilium-envoy       quay.io/cilium/cilium-envoy:v1.30.9-1737073743-40a016d11c0d863b772961ed0168eea6fe6b10a5@sha256:a69dfe0e54b24b0ff747385c8feeae0612cfbcae97bfcc8ee42a773bb3f69c88: 6
                       cilium-operator    quay.io/cilium/operator-generic:v1.16.6@sha256:13d32071d5a52c069fb7c35959a56009c6914439adc73e99e098917646d154fc: 2