helm: Use k8s `EnvVar` for `proxy.ExperimentalEnv` (#11923)

PR #11874 introduced a `proxy.ExperimentalEnv` setting, allowing
arbitrary name+value environment variables on proxies. This name+value
pairing was a subset of k8s' environment variables, specifically, it did
not allow for `valueFrom.configMapKeyRef` and related fields. PR #11908
introduced this pattern in the ControlPlane containers.

Modify `proxy.ExperimentalEnv` to behave identically to k8s' native
`EnvVar`, allowing settings such as:
```
--set proxy.experimentalEnv[0].name=LINKERD2_PROXY_DEFROBINATION
--set proxy.experimentalEnv[0].valueFrom.configMapKeyRef.key=extreme-key
--set proxy.experimentalEnv[0].valueFrom.configMapKeyRef.name=extreme-config
```

Context:
https://github.com/linkerd/linkerd2/pull/11908#issuecomment-1888945793

Signed-off-by: Andrew Seigner <siggy@buoyant.io>

charts/partials/templates/_proxy.tpl

@@ -160,9 +160,8 @@ be used in other contexts.
 - name: LINKERD2_PROXY_SHUTDOWN_GRACE_PERIOD
   value: {{.Values.proxy.shutdownGracePeriod | quote}}
 {{ end -}}
-{{ range $k, $v := (.Values.proxy.experimentalEnv) -}}
-- name: {{ $k }}
-  value: {{ $v | quote }}
+{{ if .Values.proxy.experimentalEnv -}}
+{{ toYaml .Values.proxy.experimentalEnv }}
 {{ end -}}
 image: {{.Values.proxy.image.name}}:{{.Values.proxy.image.version | default .Values.linkerdVersion}}
 imagePullPolicy: {{.Values.proxy.image.pullPolicy | default .Values.imagePullPolicy}}

pkg/charts/linkerd2/values.go

@@ -123,7 +123,7 @@ type (
 		StartupProbe                         *StartupProbe    `json:"startupProbe"`
 		Control                              *ProxyControl    `json:"control"`
 
-		ExperimentalEnv map[string]string `json:"experimentalEnv"`
+		ExperimentalEnv []corev1.EnvVar `json:"experimentalEnv"`
 	}
 
 	ProxyControl struct {