feat(api): resolve provider class dynamically via the SDK resolver

[StylusFrost]

Context

return_prowler_provider hardcoded a 16-arm match over the known provider types (with per-provider imports and a closed type union), raising ValueError for anything else. This blocked external/entry-point providers from resolving in the API.

Stacked on #11399 (T2). Depends on the SDK resolver from #11398 (T1).

Description

• Replace the match with Provider.get_class(provider.provider) (the SDK public resolver). Any provider the SDK discovers — built-in or external entry-point — now resolves without a per-provider branch.
• get_class raises ImportError for unknown providers; it is wrapped into the existing ValueError contract.
• Drop the closed provider-type union return annotations and the TYPE_CHECKING provider imports; return_prowler_provider -> type, initialize_prowler_provider -> SDKProvider.

Steps to review

• return_prowler_provider unit tests: built-in resolution (16 parametrized) + external resolution via the resolver + unknown → ValueError.
• Docker (SDK pinned to the T1 branch + an external local-template provider installed): POST /api/v1/providers/{id}/connection no longer fails with Provider type ... not supported; resolution succeeds and the task completes.

Checklist

• All issue/task requirements work as expected on the API
• Ensure new entries are added to api/CHANGELOG.md

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.