Make libsodium optional

Summary:
Fizz uses libsodium only for `X25519` and random bytes, which the OpenSSL it already requires provides (`EVP_PKEY_X25519` and `RAND_bytes`), yet every project that uses Fizz has to supply a second crypto library. Fizz's package config looks up Sodium without installing `FindSodium.cmake`, so Thrift installs one just for Fizz; Thrift's build compiles libsodium from source when it isn't installed but can't on Windows, where users have to install it separately; and the Fizz and Thrift getdeps workflows fetch and build it.

Make libsodium optional:
- `openssl::X25519KeyExchange` implements `X25519` with `EVP_PKEY_X25519`. `MultiBackendFactory` is the only place that picks the `X25519` implementation: libsodium's when Fizz is built with it, OpenSSL's otherwise. It uses it for `NamedGroup::x25519` and the hybrid post-quantum groups, and `MultiBackendFactory::makeX25519KeyExchange()` creates it from a private key for `FizzUtil`'s ECH keys and key pair generation. The tool's built-in ECH key and tests use `openssl::X25519KeyExchange`.
- `fizz::random()` in `fizz/crypto/Random.h` fills a buffer with random bytes from libsodium, or from `RAND_bytes` in builds without it. `MultiBackendFactory::makeRandomBytes` and `RandomGenerator` use it, so neither depends on libsodium headers. `useMockRandom()` replaces its source through the test-only `detail::setRandomForTesting()` instead of libsodium's hook.
- CMake builds Fizz with libsodium if it finds it, and only then compiles the libsodium backend, links libsodium, installs its headers and looks it up in the package config. `FindSodium` no longer defines a `sodium` target when it finds nothing, and on a platform or MSVC architecture it doesn't support, such as ARM64, it reports Sodium as not found instead of failing.

Internal builds still take `X25519` in `MultiBackendFactory` and `FizzUtil`, and all random bytes, from libsodium.

Reviewed By: mingtaoy

Differential Revision: D129117795

fbshipit-source-id: 301002f13112f0791ae8873b29f2f5e99ce13e39

Author: vitaut

build/fbcode_builder/CMake/FindSodium.cmake

@@ -114,14 +114,16 @@ elseif (WIN32)
         )
         string(REGEX REPLACE ".*ARCH_VALUE ([a-zA-Z0-9_]+).*" "\\1" _TARGET_ARCH "${_COMPILATION_LOG}")
 
-        # construct library path
+        # construct library path; on other architectures sodium is not found
+        set(_PLATFORM_PATH "")
         if (_TARGET_ARCH STREQUAL "x86_32")
-            string(APPEND _PLATFORM_PATH "Win32")
+            set(_PLATFORM_PATH "Win32")
         elseif(_TARGET_ARCH STREQUAL "x86_64")
-            string(APPEND _PLATFORM_PATH "x64")
-        else()
-            message(FATAL_ERROR "the ${_TARGET_ARCH} architecture is not supported by Findsodium.cmake.")
+            set(_PLATFORM_PATH "x64")
         endif()
+    endif()
+
+    if (MSVC AND _PLATFORM_PATH)
         string(APPEND _PLATFORM_PATH "/$$CONFIG$$")
 
         if (MSVC_VERSION LESS 1900)
@@ -162,7 +164,7 @@ elseif (WIN32)
             set(CMAKE_FIND_LIBRARY_SUFFIXES ${CMAKE_FIND_LIBRARY_SUFFIXES_BCK})
         endif()
 
-    elseif(_GCC_COMPATIBLE)
+    elseif(NOT MSVC AND _GCC_COMPATIBLE)
         if (sodium_USE_STATIC_LIBS)
             find_library(sodium_LIBRARY_DEBUG libsodium.a
                 HINTS ${sodium_DIR}
@@ -196,15 +198,8 @@ elseif (WIN32)
                 PATH_SUFFIXES bin
             )
         endif()
-    else()
-        message(FATAL_ERROR "this platform is not supported by FindSodium.cmake")
     endif()
 
-
-########################################################################
-# unsupported
-else()
-    message(FATAL_ERROR "this platform is not supported by FindSodium.cmake")
 endif()
 
 
@@ -248,6 +243,10 @@ if (WIN32)
     mark_as_advanced(sodium_DLL_RELEASE)
 endif()
 
+if(NOT Sodium_FOUND)
+    return()
+endif()
+
 # create imported target
 if(sodium_USE_STATIC_LIBS)
     set(_LIB_TYPE STATIC)