feat(providers): add support for custom vendors (#74)

* feat(providers): add support for custom vendors

Signed-off-by: Aaron Pham <contact@aarnphm.xyz>

* fix: override configuration not setup

Signed-off-by: Aaron Pham <contact@aarnphm.xyz>

---------

Signed-off-by: Aaron Pham <contact@aarnphm.xyz>

README.md

@@ -258,6 +258,99 @@ lua_ls = {
 
 Then you can set `dev = true` in your `lazy` config for development.
 
+## Custom Providers
+
+To add support for custom providers, one add `AvanteProvider` spec into `opts.vendors`:
+
+```lua
+{
+  provider = "my-custom-provider", -- You can then change this provider here
+  vendors = {
+    ["my-custom-provider"] = {...}
+  },
+  windows = {
+    wrap_line = true,
+    width = 30, -- default % based on available width
+  },
+  --- @class AvanteConflictUserConfig
+  diff = {
+    debug = false,
+    autojump = true,
+    ---@type string | fun(): any
+    list_opener = "copen",
+  },
+}
+
+```
+
+A custom provider should following the following spec:
+
+```lua
+---@type AvanteProvider
+{
+  endpoint = "https://api.openai.com/v1/chat/completions", -- The full endpoint of the provider
+  model = "gpt-4o", -- The model name to use with this provider
+  api_key_name = "OPENAI_API_KEY", -- The name of the environment variable that contains the API key
+  --- This function below will be used to parse in cURL arguments.
+  --- It takes in the provider options as the first argument, followed by code_opts retrieved from given buffer.
+  --- This code_opts include:
+  --- - question: Input from the users
+  --- - code_lang: the language of given code buffer
+  --- - code_content: content of code buffer
+  --- - selected_code_content: (optional) If given code content is selected in visual mode as context.
+  ---@type fun(opts: AvanteProvider, code_opts: AvantePromptOptions): AvanteCurlOutput
+  parse_curl_args = function(opts, code_opts) end
+  --- This function will be used to parse incoming SSE stream
+  --- It takes in the data stream as the first argument, followed by opts retrieved from given buffer.
+  --- This opts include:
+  --- - on_chunk: (fun(chunk: string): any) this is invoked on parsing correct delta chunk
+  --- - on_complete: (fun(err: string|nil): any) this is invoked on either complete call or error chunk
+  --- - event_state: SSE event state.
+  ---@type fun(data_stream: string, opts: ResponseParser): nil
+  parse_response_data = function(data_stream, opts) end
+}
+```
+
+<details>
+<summary>Full working example of perplexity</summary>
+
+```lua
+vendors = {
+  ---@type AvanteProvider
+  perplexity = {
+    endpoint = "https://api.perplexity.ai/chat/completions",
+    model = "llama-3.1-sonar-large-128k-online",
+    api_key_name = "PPLX_API_KEY",
+    --- this function below will be used to parse in cURL arguments.
+    parse_curl_args = function(opts, code_opts)
+      local Llm = require "avante.llm"
+      return {
+        url = opts.endpoint,
+        headers = {
+          ["Accept"] = "application/json",
+          ["Content-Type"] = "application/json",
+          ["Authorization"] = "Bearer " .. os.getenv(opts.api_key_name),
+        },
+        body = {
+          model = opts.model,
+          messages = Llm.make_openai_message(code_opts), -- you can make your own message, but this is very advanced
+          temperature = 0,
+          max_tokens = 8192,
+          stream = true, -- this will be set by default.
+        },
+      }
+    end,
+    -- The below function is used if the vendors has specific SSE spec that is not claude or openai.
+    parse_response_data = function(data_stream, opts)
+      local Llm = require "avante.llm"
+      Llm.parse_openai_response(data_stream, opts)
+    end,
+  },
+},
+```
+
+</details>
+
 ## License
 
 avante.nvim is licensed under the Apache License. For more details, please refer to the [LICENSE](./LICENSE) file.