refactoring: move common shard logic

[AndrewChubatiuk]

moved common sharding logic into build package
extracted from #1554

[f41gh7]

LGTM

[f41gh7]

Please fix test:

 [FAILED] Timed out after 80.001s.
  Expected success, but got an error:
      <*errors.errorString | 0xc000fe26d0>: 
      not expected object status="failed", reason="cannot reconcile sts for vmagent(0): cannot handle rolling-update on sts: vmagent-stateful-shard-0, err: actual pod count: 0 less than needed: 1, possible statefulset misconfiguration",conditions=
      {
          s: "not expected object status=\"failed\", reason=\"cannot reconcile sts for vmagent(0): cannot handle rolling-update on sts: vmagent-stateful-shard-0, err: actual pod count: 0 less than needed: 1, possible statefulset misconfiguration\",conditions=",
      }
  In [It] at: /home/runner/work/operator/operator/test/e2e/vmagent_test.go:431 @ 11/07/25 09:05:17.698
------------------------------
•••••
------------------------------
• [FAILED] [293.420 seconds]
test vmanomaly Controller e2e vmanomaly should update exist vmanomaly [It] by switching to shard mode [vm, anomaly, enterprise]
/home/runner/work/operator/operator/test/e2e/vmanomaly_test.go:329

  [FAILED] Timed out after 240.001s.
  Expected success, but got an error:
      <*errors.errorString | 0xc0003879b0>: 
      not expected object status="failed", reason="cannot handle rolling-update on sts: vmanomaly-shard-0, err: actual pod count: 0 less than needed: 1, possible statefulset misconfiguration\ncannot handle rolling-update on sts: vmanomaly-shard-1, err: actual pod count: 0 less than needed: 1, possible statefulset misconfiguration",conditions=
      {
          s: "not expected object status=\"failed\", reason=\"cannot handle rolling-update on sts: vmanomaly-shard-0, err: actual pod count: 0 less than needed: 1, possible statefulset misconfiguration\\ncannot handle rolling-update on sts: vmanomaly-shard-1, err: actual pod count: 0 less than needed: 1, possible statefulset misconfiguration\",conditions=",
      }

[f41gh7]

Thanks for contribution!