1) Change return type to WvCasStatus for functions in wv_cas_types.cc.

2) Add a binary wv_cas_types_example.
3) Surface wv_cas_key_fetcher *source code* to partner to serve as an example of how they would make a HTTP request to acquire an entitlement key from license server.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=229953562

example/BUILD

@@ -18,7 +18,9 @@ filegroup(
     srcs = [
         "test_ecmg_messages.h",
         "wv_cas_ecm_example.cc",
+        "wv_cas_types_example.cc",
         ":wv_cas_ecm_example",
+        ":wv_cas_types_example",
     ],
 )
 
@@ -32,10 +34,16 @@ cc_library(
     hdrs = ["test_ecmg_messages.h"],
 )
 
+cc_library(
+    name = "test_emmg_messages",
+    hdrs = ["test_emmg_messages.h"],
+)
+
 cc_binary(
     name = "wv_cas_ecm_example",
     srcs = ["wv_cas_ecm_example.cc"],
     deps = [
+        "//base",
         "@abseil_repo//absl/base:core_headers",
         "//media_cas_packager_sdk/public:wv_cas_ecm",
         "//media_cas_packager_sdk/public:wv_cas_types",
@@ -52,3 +60,13 @@ cc_binary(
         "//protos/public:media_cas_encryption_proto",
     ],
 )
+
+cc_binary(
+    name = "wv_cas_types_example",
+    srcs = ["wv_cas_types_example.cc"],
+    deps = [
+        "//base",
+        "@abseil_repo//absl/base:core_headers",
+        "//media_cas_packager_sdk/public:wv_cas_types",
+    ],
+)