feat(loadbalancer): add labels - #1827

Description

relates to #1825

Adds labels to stackit_loadbalancer (resource and data source). The Network Load Balancer API already accepts labels on create and update (CreateLoadBalancerPayload.Labels, UpdateLoadBalancerPayload.Labels).

The schema and validators follow stackit_application_load_balancer, with two differences:

• Label values may be empty (LengthBetween(0, 63)), as documented for the NLB API.
• An empty label map returned by the API is mapped to null, so a configuration without labels does not end up with {} in state.

This is the second part of #1825; the health check options are in #1826. Both PRs touch the same files, so whichever is merged second will need a rebase — happy to do that.

Testing

• Unit tests extended for toCreatePayload, toUpdatePayload and mapFields, plus a new Test_mapLabels.
• TestAccLoadBalancerResourceMax extended with labels (create, data source and update step changing the label value) and run against a STACKIT project: PASS.
• make fmt, make lint, make test, make generate-docs pass locally.
• Not covered by a test: removing labels from an existing load balancer. The update call sends the full load balancer, so omitting labels should clear them, but I did not verify this against the API.

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)