Clarify sharding label management

[weyfonk]

Fleet uses two labels for its current sharding implementation:

• controllers are labelled with shard=<shard-id>
• sharded resources bear label fleet.cattle.io/shard=<shard-id>

We should:

• clarify how we want to name(space) those labels, making them consistent across Fleet, eg:
  • shard.fleet.cattle.io/id=<shard-id>
• expose those labels in Fleet's public API, to enable users to consume them.

Note: info on how Flux handles sharding.

Follow-up on #1740.

[manno]

controller label:
fleet.cattle.io/shard-id=<shard-id>

sharded resources bear label:
fleet.cattle.io/shard-ref=<shard-id>

[kkaempf]

considered done