HPCC-9090 Add base 64 encode/decode functions to the ecllibrary

This new feature contains these implementations:

1. Add a new implementation of JBASE64_Decode which takes a pointer to input
   string and the length of it.

2. Add wrapper functions rtlBase64Encode() and rtlBase64Decode() into eclrtl
   (.hpp and .cpp) to call Base 64 encode/decode functions (implemented in
   jutil.cpp).

3. Add implementation to rtlMalloc() and rtlRealloc() functions to handle
   memory allocation fault. Added macros to handle logging exact filename
   and line number if memory allocate fails.

4. Add external references and exports for functions implemented in 1. into
   Str.ecl.

5. Add test code TestBase64Codec.ecl into ecllibrary/teststd/str/ directory.

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>