refactor: Add paychan namespace and update related tests (#5840)

This change adds a paychan namespace to the TestHelpers and implementation files, improving organization and clarity. Additionally, it updates the AMM test to use the new `paychan::create` function for payment channel creation.

Co-authored-by: Bart Thomee <11445373+bthomee@users.noreply.github.com>

src/test/app/AMM_test.cpp

@@ -3666,7 +3666,7 @@ private:
             auto const settleDelay = 100s;
             NetClock::time_point const cancelAfter =
                 env.current()->info().parentCloseTime + 200s;
-            env(create(
+            env(paychan::create(
                     carol,
                     ammAlice.ammAccount(),
                     XRP(1'000),