[cdc_rsync] Add initial support for Windows (#51)

Adds a ServerArch class whose job it is to encapsulate differences
between Windows and Linux cdc_rsync_servers. It detects the type
based on a heuristic in the destination path. This is not fool proof
and will probably require further work, like falling back to the other
type if the detected one doesn't work.

Uses the ServerArch class to determine the different commands to start
the server and to deploy the server.

Note that the functionality is not well tested on Windows yet, but
copying plain files works.

cdc_rsync/BUILD

@@ -79,6 +79,7 @@ cc_library(
         ":file_finder_and_sender",
         ":parallel_file_opener",
         ":progress_tracker",
+        ":server_arch",
         ":zstd_stream",
         "//cdc_rsync/base:cdc_interface",
         "//cdc_rsync/base:message_pump",
@@ -172,6 +173,28 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "server_arch",
+    srcs = ["server_arch.cc"],
+    hdrs = ["server_arch.h"],
+    deps = [
+        "//common:path",
+        "//common:remote_util",
+        "@com_google_absl//absl/strings",
+    ],
+)
+
+cc_test(
+    name = "server_arch_test",
+    srcs = ["server_arch_test.cc"],
+    deps = [
+        ":server_arch",
+        "//common:test_main",
+        "@com_google_absl//absl/strings",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
 cc_library(
     name = "zstd_stream",
     srcs = ["zstd_stream.cc"],