feat(sdk): add Provider.get_class dynamic provider resolver

[StylusFrost]

Context

Part of the Dynamic Providers (API) epic (PROWLER-1472) — task PROWLER-1771, the first slice toward decoupling the API from hardcoded provider types.

The SDK already discovers providers dynamically via entry points (PROWLER-1391, #10700), but the class-resolution logic ("given a provider name, return its provider class") lived inline inside init_global_provider (which sets global state and calls sys.exit) and get_providers_help_text. There was no public, side-effect-free API the Django API could call, forcing it to keep a hardcoded match over provider types.

Stacked PR: this targets PROWLER-1391-provider-contract-dynamic-discovery (#10700), not master, because it builds on the dynamic-discovery helpers introduced there. Merge #10700 first.

Description

• Add public Provider.get_class(provider) resolving a provider class by name for both built-in (direct import) and external entry-point providers. Side-effect-free: no set_global_provider, no sys.exit; the caller decides error handling (CLI exits, the API can return HTTP 400).
• Refactor init_global_provider to use it, preserving CLI behavior: a built-in's missing transitive dependency still logs critical + sys.exit(1). The collision warning (built-in wins over a same-named plug-in) stays here, at the point of use, so prowler --help no longer emits it spuriously.
• Refactor get_providers_help_text to use it.

Steps to review

• uv run pytest tests/providers/external/test_dynamic_provider_loading.py -v (97 passing, incl. new TestGetClass).
• Provider.get_class("aws") returns AwsProvider; an external entry-point provider resolves to its class; an unknown name raises ImportError (never sys.exit).
• prowler --help emits no spurious collision warnings.

Checklist

• Review if the code is being covered by tests.
• Review if code is being documented following the Google Python style guide.
• Review if backport is needed.
• Review if is needed to change the Readme.md
• Ensure new entries are added to CHANGELOG.md, if applicable.

SDK/CLI

• Are there new checks included in this PR? No

License

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