Add customPlugins and unsupportedBuiltInPlugins sections to spire-server (#198)

This patch enables end users to configure external plugins in the
spire-server config. Unsupported internal plugins are not able to be
set.

---------

Signed-off-by: Kevin Fox <[email protected]>
Signed-off-by: kfox1111 <[email protected]>
Co-authored-by: Edwin Buck <[email protected]>
Co-authored-by: Faisal Memon <[email protected]>

charts/spire/templates/NOTES.txt

@@ -1 +1,8 @@
 Installed {{ .Chart.Name }}…
+{{- $up := (index .Values "spire-server").unsupportedBuiltInPlugins }}
+{{- $s := add (len $up.keyManager) (len $up.nodeAttestor) (len $up.upstreamAuthority) (len $up.notifier) }}
+{{- if gt $s 0 }}
+
+Warning:
+  You are using an unsupported plugin. Functionality of this release and future upgrades are not guaranteed to work smoothly.
+{{- end }}