[encoding] avoid the use of using for Big/LittleEndian functions (#9621)

This commit refactors the code to eliminate the use of `using` for
`BigEndian` or `LittleEndian` functions like `HostSwap` and `ReadUint`.
As these functions are frequently used in header files, using the
direct namespace enhances code safety by mitigating potential conflicts
arising from the order of included headers in the absence of explicit
`using` declarations. Generally, avoiding the `using` keyword in headers
is considered a recommended practice.

Additionally, this commit removes the `Encoding` namespace to shorten
the full function names.

tests/unit/test_ip_address.cpp

@@ -826,7 +826,6 @@ void TestIp4Ip6Translation(void)
 
 void TestIp4Cidr(void)
 {
-    using Encoding::BigEndian::HostSwap32;
     struct TestCase
     {
         const char    *mNetwork;