sdk: introduce Spice SDK (#676)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Adds a flat-C, headers-only library for writing DLL plugins with spice.

This is meant to be an alternative to writing applications over spice
API which doesn't suit some scenarios like writing custom
high-performance I/O modules.

All you have to do is:

1. Create a 32-bit or 64-bit DLL with `spice_sdk_entry_point` as a
DLLExport
1. Include `spicesdk.h`, and optionally `spicesdk_io.h` (see
`extras\sdk\include`)
1. `spice_sdk_entry_point` will be called when hooks get initialized.
Call `init` with valid parameters.
1. When `init` returns, `SPICE_SDK_V0` will be filled out with a series
of function pointers; your DLL can now start calling into it.
1. Compile your DLL and add it as a DLL hook (`-k`) in spice when
launching the game.

See `v0_cpp.cpp` for an example.

V0 implements the following functions:

```
    log;

    get_game_info;
    get_avs_info;

    get_button;
    set_button;

    get_analog;
    set_analog;

    get_light;
    set_light;

    set_touch;
    clear_touch;

    insert_card;
    set_keypad;
```

V0 is focused on providing common utility functions needed in a lot of
DLL hooks (get AVS info, log messages to file), and implementing I/O.

In the future, we could have more interesting things like:

* scanning and modifying memory 
* installing DLL hooks
* having direct hooks into I/O emulation modules
* getting full tape LEDs
* having a callback into DX9 `Present` call
* drawing an ImGui window

More detailed documentation is coming soon in form of a wiki page.

## Testing
See included sample DLL.

src/spice2x/launcher/launcher.cpp

@@ -98,6 +98,7 @@
 #include "rawinput/rawinput.h"
 #include "rawinput/touch.h"
 #include "reader/reader.h"
+#include "sdk/sdk.h"
 #include "stubs/stubs.h"
 #include "touch/touch.h"
 #include "util/cpuutils.h"
@@ -2358,6 +2359,7 @@ int main_implementation(int argc, char *argv[]) {
                 });
         } else {
             bt5api_hook(module);
+            sdk::register_sdk_hooks(hook, module);
         }
     }
 
@@ -2392,6 +2394,9 @@ int main_implementation(int argc, char *argv[]) {
         bt5api_init();
     }
 
+    // init SDK
+    sdk::init_sdk_modules();
+
     // API
     if (api_enable || std::min(api_serial_port.size(), api_serial_baud.size()) > 0) {
         API_CONTROLLER = std::make_unique<api::Controller>(api_port, api_pass, api_pretty);
@@ -2499,6 +2504,8 @@ int main_implementation(int argc, char *argv[]) {
         bt5api_dispose();
     }
 
+    sdk::fini_sdk_modules();
+
     // stop raw input
     RI_MGR.reset();