internal/rpmmd: RepoConfig baseurl change

Update the internal RepoConfig object to
accept a slice of baseurls rather than a
single field. This change was needed to
align RepoConfig with the dnf spec [1].

Additionally, this change adds custom json
marshal and unmarshal functions to ensure
backwards compatibility with older workers.
Add json tags to the internal rpmmd config
since this is serialized in dnfjson.
Add unit tests to check the serialization
is okay.

[1] See dnf.config

internal/client/unit_test.go

@@ -75,7 +75,7 @@ func executeTests(m *testing.M) int {
 	rr := reporegistry.NewFromDistrosRepoConfigs(rpmmd.DistrosRepoConfigs{
 		test_distro.TestDistroName: {
 			test_distro.TestArchName: {
-				{Name: "test-system-repo", BaseURL: "http://example.com/test/os/test_arch"},
+				{Name: "test-system-repo", BaseURLs: []string{"http://example.com/test/os/test_arch"}},
 			},
 		},
 	})