Implement hook_dfp_tag_alter()

I don't think hook_dfp_tag_load_alter(), makes sense in D10.

Done. Please review the provided MR!

Thanks @Grevil! Looks and works great!

Current MR as a static patch until it is merged.

Just found an issue with this implementation: 🐛 Tag altering is not represented in caching (= wrong ads targeting shown) Needs work

While it's super important to have the flexibility this hook provides, it doesn't care for caching.
I'm not even sure if the issue described doesn't also happen in other cases already, but at least for this MR the differences in targeting for the same tag need to be taken into account!

We may decide to merge that other issue in here and fix it here. But first we need to find a correct solution.

Any ideas everyone here?