feat: anthropic provider can take options.headers

Lets us set the beta 8k output token header as an option
`anthropic-beta: "max-tokens-3-5-sonnet-2024-07-15"`

lua/model/providers/anthropic.lua

@@ -3,15 +3,17 @@ local sse = require('model.util.sse')
 
 ---@type Provider
 local M = {
-  request_completion = function(handler, params)
+  request_completion = function(handler, params, options)
+    options = options or {}
+
     return sse.curl_client({
       url = 'https://api.anthropic.com/v1/messages',
-      headers = {
+      headers = vim.tbl_extend('force', {
         ['Content-Type'] = 'application/json',
         ['x-api-key'] = util.env('ANTHROPIC_API_KEY'),
         ['anthropic-beta'] = 'messages-2023-12-15',
         ['anthropic-version'] = '2023-06-01',
-      },
+      }, options.headers or {}),
       body = vim.tbl_deep_extend('force', {
         max_tokens = 1024, -- required field
       }, params, { stream = true }),
@@ -24,7 +26,9 @@ local M = {
         elseif msg.event == 'content_block_delta' then
           handler.on_partial(data.delta.text)
         elseif msg.event == 'message_delta' then
-          -- util.show(data.usage.output_tokens)
+          util.show(data.usage.output_tokens, 'output tokens')
+          -- else
+          --   util.show(msg, 'msg')
         end
       end,
       on_error = handler.on_error,