base: add base64.cc

Add a dedicated base64 translation unit for both encoding
and decoding. Will be used by follow-up CLs.
Also refactor existing code currently using base64
from string_utils.cc.

Bug: 205274609
Change-Id: I8ce97dd09fc1707c9989c0f148bd972b805d4b49
diff --git a/BUILD b/BUILD
index 6422ad6..c69e93d 100644
--- a/BUILD
+++ b/BUILD
@@ -335,6 +335,7 @@
     name = "include_perfetto_ext_base_base",
     srcs = [
         "include/perfetto/ext/base/android_utils.h",
+        "include/perfetto/ext/base/base64.h",
         "include/perfetto/ext/base/circular_queue.h",
         "include/perfetto/ext/base/container_annotations.h",
         "include/perfetto/ext/base/crash_keys.h",
@@ -640,6 +641,7 @@
     name = "src_base_base",
     srcs = [
         "src/base/android_utils.cc",
+        "src/base/base64.cc",
         "src/base/crash_keys.cc",
         "src/base/ctrl_c_handler.cc",
         "src/base/event_fd.cc",