Add extra initContainers, containers, volumes to agent and server

With plugin support, agents and servers need more customization.
This patch enables initContainers, extraContainers, extraVolumes
and extraVolumeMounts to be added to those services.

Signed-off-by: Kevin Fox <[email protected]>

charts/spire/charts/spire-agent/README.md

@@ -16,6 +16,9 @@ A Helm chart to install the SPIRE agent.
 |-----|------|---------|-------------|
 | bundleConfigMap | string | `"spire-bundle"` |  |
 | clusterName | string | `"example-cluster"` |  |
+| extraContainers | list | `[]` |  |
+| extraVolumeMounts | list | `[]` |  |
+| extraVolumes | list | `[]` |  |
 | fullnameOverride | string | `""` |  |
 | healthChecks.port | int | `9980` | override the host port used for health checking |
 | image.pullPolicy | string | `"IfNotPresent"` |  |
@@ -23,6 +26,7 @@ A Helm chart to install the SPIRE agent.
 | image.repository | string | `"spiffe/spire-agent"` |  |
 | image.version | string | `""` |  |
 | imagePullSecrets | list | `[]` |  |
+| initContainers | list | `[]` |  |
 | logLevel | string | `"info"` |  |
 | nameOverride | string | `""` |  |
 | nodeSelector | object | `{}` |  |