gh-35346: Do not require a multiplicative generator for finite field nth_root
### 📚 Description
This patch avoids computing a multiplicative generator in `nth_root` for
finite fields, which usually requires factoring the order of the
multiplicative group, which can be very expensive. The following
example, included in doctests of this patch, currently does not complete
in finite time (instead of a few ms after the change):
```
sage: p = 2^1024 + 643
sage: a = GF(p, proof=False)(3)**(29*12345)
sage: a.nth_root(29)
```
The issue is well-known to various users. A previous proposal, Trac
ticket #28585 was suggesting switching to Adleman-Manders-Miller
entirely to fix the same issue.
This patch instead performs minimal changes, keeps the Johnston's
algorithm, only modifying how the value of `g^h` is computed (it seems
legit to still call it Johnston's algorithm because it processes
`r^k`-th roots in a single discrete log, using the formula from the
article), using the existing `zeta` method.
### 📝 Checklist
- [x] I have made sure that the title is self-explanatory and the
description concisely explains the PR.
- [x] I have linked an issue or discussion.
- [x] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/35346
Reported by: Rémy Oudompheng
Reviewer(s): Vincent Delecroix
