feat: support customizing system_prompt in the configuration (#1527)

lua/avante/config.lua

@@ -34,6 +34,7 @@ M._defaults = {
   -- For most providers that we support we will determine this automatically.
   -- If you wish to use a given implementation, then you can override it here.
   tokenizer = "tiktoken",
+  system_prompt = nil,
   rag_service = {
     enabled = false, -- Enables the rag service, requires OPENAI_API_KEY to be set
     host_mount = os.getenv("HOME"), -- Host mount path for the rag service (docker will mount this path)

lua/avante/llm.lua

@@ -128,6 +128,10 @@ function M.generate_prompts(opts)
 
   local system_prompt = Path.prompts.render_mode(mode, template_opts)
 
+  if Config.system_prompt ~= nil and Config.system_prompt ~= "" and Config.system_prompt ~= "null" then
+    system_prompt = system_prompt .. "\n\n" .. Config.system_prompt
+  end
+
   ---@type AvanteLLMMessage[]
   local messages = {}