shard_param validation pattern

k8s uses re2, which does not support lookaround, so this rather
complicated pattern is what I came up with to forbid vcl and
vk8s prefixes.

charts/viking-controller/crds/backendcfg-crd.yaml

@@ -136,6 +136,7 @@ spec:
                     pattern: '^\d+(\.\d+)?(ms|[smhdwy])$'
                   shard_param:
                     type: string
+                    pattern: '^([a-uw-zA-UW-Z]|v[-abd-jl-zABD-J-ZABD-JL-Z0-9_]|vc[-a-km-zA-KM-Z0-9_]|vk[-a-zA-Z0-79_]|vk8[-a-rt-zA-Rt-z0-9_])[-a-zA-Z0-9_]*$'
               tls:
                 type: object
                 properties:

docs/ref-backend-cfg.md

@@ -277,6 +277,11 @@ used from custom VCL to control the behaviour of the
 director. Multiple directors can share the same ``shard_param``
 object.
 
+The ``shard_param`` name can not start with ``vcl`` or ``vk8s`` and
+otherwise must only contain characters valid for vcl synbols: It must
+start with an alphabetical character and only contain alphanumerical
+characters plus ``-`` and ``_``.
+
 For example:
 
 ```