pw_base64: Module for Base64 encoding and decoding

Provides C and C++ functions for encoding, decoding, and checking the
validity of Base64 data.

Change-Id: I784460d802c68b01ecee5a1778626f289685a871

modules.gni

diff --git a/modules.gni b/modules.gni
index 348e5e0..59faf73 100644
--- a/modules.gni
+++ b/modules.gni
@@ -16,6 +16,7 @@
 # allows modules to be moved or swapped out without breaking existing builds.
 # All module variables are prefixed with dir_.
 
+dir_pw_base64 = "$dir_pigweed/pw_base64"
 dir_pw_bloat = "$dir_pigweed/pw_bloat"
 dir_pw_build = "$dir_pigweed/pw_build"
 dir_pw_cpu_exception = "$dir_pigweed/pw_cpu_exception"