[lldb-server] Add accelerator plugin infrastructure for debugging hardware accelerators like gpus (#198907)

This is the first patch of many related to
https://discourse.llvm.org/t/upstreaming-basic-support-for-accelerators/89827/6

### What this patch adds

  - **`LLDBServerAcceleratorPlugin`** base class in
`source/Plugins/Process/gdb-remote/` so accelerator can implement the
own plugin
  - **`accelerator-plugins+`** feature in `qSupported` response, only
    advertised when at least one plugin is installed
- **`jAcceleratorPluginInitialize`** GDB remote packet and its
implementation in handlers, request and response.
- **`AcceleratorActions`** struct so every plugin can return the actions
that needs to be on the initilaize. in the future we will extend this
install breakpoints etc.
  - **Mock accelerator plugin** for testing, gated by CMake option
    `LLDB_ENABLE_MOCK_ACCELERATOR_PLUGIN` (default OFF)
  - **Tests** that connect to a real lldb-server, verify
    `accelerator-plugins+` in `qSupported`, send
    `jAcceleratorPluginInitialize`, and validate the JSON response

  ### Design decisions
  - CMake option defaults to OFF so normal builds are unaffected
  - Tests skip automatically when the plugin is not compiled in

GitOrigin-RevId: 3d24f9acc92bc3288efa8e17eee6ae41d694677f