Add SSWS API-key Authorization scheme

Okta API tokens authenticate as "Authorization: SSWS <token>", a
scheme none of the existing API-key modes (Bearer, custom header,
Basic) can express. Add Registration.APIKeyAuthScheme, plumb it onto
APIKeyConnection.Scheme, and send it via a new schemeAuthTransport.

The three API-key presentations (BasicAuth, Header, Scheme) are
mutually exclusive; Register rejects setting more than one so a
misconfiguration fails at process start rather than silently.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/connector/provider/registry_test.go

@@ -95,6 +95,20 @@ func TestRegistry_Register(t *testing.T) {
 		assert.Contains(t, err.Error(), "mutually exclusive")
 	})
 
+	t.Run("APIKeyAuthScheme and APIKeyHeader mutually exclusive", func(t *testing.T) {
+		t.Parallel()
+
+		r := provider.NewRegistry()
+		err := r.Register(&provider.Registration{
+			Provider:         coredata.ConnectorProviderSlack,
+			DisplayName:      "Slack",
+			APIKeyAuthScheme: "SSWS",
+			APIKeyHeader:     "x-api-key",
+		})
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "mutually exclusive")
+	})
+
 	t.Run("duplicate registration", func(t *testing.T) {
 		t.Parallel()