The self-sharding field in VarnishConfig now has a mandatory field
rules. This is an array of at least one object with a mandatory
field shard and an optional field conditions.

spec.self-sharding.rules[n].shard configures sharding in the fields
key, digest and primaryOnly (all optional).

spec.self-sharding.rules[n].conditions, if specified, is an array
of at least one condition, which specifies a boolean expression.
It is the same as conditions used for other contexts such as
rewrites.

The rules array is evaluated in order as an if-elsif-else sequence.
This means that if any element of the array specifies no conditions,
it MUST be the last rule in the array. In an array of length > 1,
it becomes the else case. This constraint is trivially satisfied
if there is only one rule. It is currently *not* enforced (would be
suitable for a webhook validation).

The fields probe and max2ndttl are now top-level fields under
self-sharding, both optional.