pw_tokenizer: C++ support for the Base64 format

Provide functions for encoding and decoding tokenized messages in the
standard prefixed Base64 format.

Change-Id: I8d09fddd29f5e3589228e5e1947cde11149c8233
diff --git a/pw_tokenizer/BUILD b/pw_tokenizer/BUILD
index 7fe7aa1..49c2b5c 100644
--- a/pw_tokenizer/BUILD
+++ b/pw_tokenizer/BUILD
@@ -49,6 +49,22 @@
 )
 
 pw_cc_library(
+    name = "base64",
+    srcs = [
+        "base64.cc",
+    ],
+    hdrs = [
+        "public/pw_tokenizer/base64.h",
+    ],
+    includes = ["public"],
+    deps = [
+        "//pw_base64",
+        "//pw_preprocessor",
+        "//pw_span",
+    ],
+)
+
+pw_cc_library(
     name = "decoder",
     srcs = [
         "decode.cc",
@@ -109,6 +125,16 @@
 )
 
 pw_cc_test(
+    name = "base64_test",
+    srcs = [
+        "base64_test.cc",
+    ],
+    deps = [
+        ":base64",
+    ],
+)
+
+pw_cc_test(
     name = "decode_test",
     srcs = [
         "decode_test.cc",