solver: include "solver" key in the dnfjson reply

This commit includes the used sovler in the dnfjson reply. This
is mostly information (e.g. in service logs) but also useful in
tests to ensure that the expected solver was really run.

Note that this needs https://github.com/osbuild/images/pull/723
first.

osbuild/solver/dnf.py

@@ -306,6 +306,7 @@ class DNF(SolverBase):
                 "sslclientcert": repo.sslclientcert,
             }
         response = {
+            "solver": "dnf",
             "packages": packages,
             "repos": repositories,
         }

osbuild/solver/dnf5.py

@@ -429,6 +429,7 @@ class DNF5(SolverBase):
                 "sslcacert": get_string_option(repo_cfg.get_sslcacert_option()),
             }
         response = {
+            "solver": "dnf5",
             "packages": packages,
             "repos": repositories,
         }