feat(observability): add http check resource - #1830

Description

relates to #1829

Adds stackit_observability_http_check, based on the HTTP check endpoints of the Observability API (CreateHttpCheck, ListHttpChecks, DeleteHttpCheck).

• The API has no update and no single-get operation for checks, so url, project_id and instance_id force a replacement, and Read looks the check up by ID in the list response (removed from state if the check or the instance is gone).
• On create, the ID is taken from httpCheck in the response. If that field is missing, the check is looked up by URL in httpChecks (create fails if it cannot be found unambiguously).
• Checks are unique per URL: I verified against the API that creating a check for a URL that already exists returns the existing check (same ID) instead of a new one. Two resources with the same url would therefore manage the same check, so this is stated in the resource description.
• Import is supported with project_id,instance_id,http_check_id.

The certificate check from #1829 will follow in a separate PR.

Testing

• Unit tests for mapFields, toCreatePayload, the ID lookup on create and the lookup on read.
• TestAccResourceMin (observability) extended with the HTTP check: create, import, and update (URL change → replacement). Run against a STACKIT project: PASS.
• make fmt, make lint, make test, make generate-docs pass locally.

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)