feat(observability): add metrics_relabel_configs to scrape config - #1834

Description

relates to #1832

Second part of #1832: adds metrics_relabel_configs to stackit_observability_scrapeconfig (resource and data source).

• action (validated against the API enum), regex, replacement, separator and source_labels are optional + computed: the API fills in defaults for unset values (replace, .*, $1, ;), so without Computed every apply would end in an inconsistent result. No defaults are hard-coded in the provider.
• target_label and modulus are optional; the API only returns them when set.
• modulus is an int32 in the response but a float32 in the request payloads; the provider uses Int32 and converts when building the payload.

This PR and #1833 both touch the scrape config resource and the observability acceptance test, so the second one to be merged will need a rebase — happy to do that.

Testing

• Unit tests for the mapping (including the empty case) and for the create/update payloads (unknown computed values are not sent).
• TestAccResourceMin (observability) extended with two relabel rules — one fully specified, one with only source_labels and target_label to check the API defaults — on create, data source and update. 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)