gh-35527: Improve morphisms between Drinfeld modules
    
We implement several methods for arbitrary morphisms between Drinfeld
modules.
Especially, we implement the computation of norms and characteristic
polynomials.

@ymusleh @DavidAyotte @kryzar

### 📚 Description

Here is a list of the most important added methods:
- `hom`: fast interface for creating morphisms between Drinfeld modules
- `_composition_`/`inverse`: composition and inverse of morphisms
between Drinfeld modules
- `is_isomorphic`: check whether two Drinfeld modules are isomorphic
either over the base field or over an algebraic closure
- `norm`: computation of the norm of a morphism between Drinfeld modules
- `dual_isogeny`: computation of the dual isogeny
- `characteristic_polynomial`/`charpoly`: computation of the
characteristic polynomial of an *endomorphism* between Drinfeld modules

We also implement addition of morphisms and the action of the function
field $A$; this gives the structure of left $A$-module on the set of
morphisms between two given Drinfeld modules.

### 📝 Checklist

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- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
    
URL: #35527
Reported by: Xavier Caruso
Reviewer(s): Antoine Leudière, David Ayotte, Xavier Caruso, ymusleh

Author: Release Manager

src/sage/categories/drinfeld_modules.py

@@ -768,3 +768,23 @@ def ore_polring(self):
                 True
             """
             return self.category().ore_polring()
+
+        def ore_variable(self):
+            r"""
+            Return the variable of the Ore polynomial ring of this Drinfeld module.
+
+            EXAMPLES::
+
+                sage: Fq = GF(25)
+                sage: A.<T> = Fq[]
+                sage: K.<z12> = Fq.extension(6)
+                sage: p_root = 2*z12^11 + 2*z12^10 + z12^9 + 3*z12^8 + z12^7 + 2*z12^5 + 2*z12^4 + 3*z12^3 + z12^2 + 2*z12
+                sage: phi = DrinfeldModule(A, [p_root, z12^3, z12^5])
+
+                sage: phi.ore_polring()
+                Ore Polynomial Ring in t over Finite Field in z12 of size 5^12 over its base twisted by Frob^2
+                sage: phi.ore_variable()
+                t
+
+            """
+            return self.category().ore_polring().gen()