Add <iframe> provider model and parser

What does this MR do and why?

Adds the <iframe> provider model and parser to the codebase, with no callers.

The provider model has been developed in conjuction with AppSec; see #599713 and https://gitlab.com/gitlab-com/gl-security/product-security/appsec/appsec-reviews/-/work_items/326 (staff-only), in particular https://gitlab.com/gitlab-com/gl-security/product-security/appsec/appsec-reviews/-/work_items/326#note_3306306968.

MR stack

MR 1/9 of Rework <iframe> embed configuration (#599713):

1. Add `<iframe>` provider model and parser (!254943 - merged) ⬅️ you are here
2. Add `<iframe>` provider loader and base config (!254944 - merged)
3. Convert `<iframe>` allowlist setting to provide... (!254945)
4. Draft: Move to `<iframe>` providers (!254946)
5. Draft: Remove dead `<iframe>` transform code (!254947)
6. Draft: Frontend `<iframe>` hardening (!254948)
7. Draft: Click-to-activate functionality for `<if... (!254949)
8. Draft: Admin UI for `<iframe>` providers (!254950)
9. Draft: Mark `<iframe>` provider loader and conf... (!255246)

Part of <iframe> embeds in Markdown content (#282443).

MR acceptance checklist

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