feat: support cmd: loading api keys for google search APIs (#1532)

refactor environment loading logic into Utils and use it for existing
provider loading, and web search where appropriate.

lua/avante/utils/environment.lua

@@ -0,0 +1,64 @@
+local Utils = require("avante.utils")
+
+---@class avante.utils.environment
+local M = {}
+
+---@private
+---@type table<string, string>
+M.cache = {}
+
+---Parse environment variable using optional cmd: feature with an override fallback
+---@param key_name string
+---@param override? string
+---@return string | nil
+function M.parse(key_name, override)
+  if key_name == nil then error("Requires key_name") end
+
+  local cache_key = type(key_name) == "table" and table.concat(key_name, "__") or key_name
+
+  if M.cache[cache_key] ~= nil then return M.cache[cache_key] end
+
+  local cmd = type(key_name) == "table" and key_name or key_name:match("^cmd:(.*)")
+
+  local value = nil
+
+  if cmd ~= nil then
+    if override ~= nil and override ~= "" then
+      value = os.getenv(override)
+      if value ~= nil then
+        M.cache[cache_key] = value
+        return value
+      end
+    end
+
+    if type(cmd) == "string" then cmd = vim.split(cmd, " ", { trimempty = true }) end
+
+    Utils.debug("running command:", cmd)
+    local exit_codes = { 0 }
+    local ok, job_or_err = pcall(vim.system, cmd, { text = true }, function(result)
+      Utils.debug("command result:", result)
+      local code = result.code
+      local stderr = result.stderr or ""
+      local stdout = result.stdout and vim.split(result.stdout, "\n") or {}
+      if vim.tbl_contains(exit_codes, code) then
+        value = stdout[1]
+        M.cache[cache_key] = value
+      else
+        Utils.error("failed to get key: (error code" .. code .. ")\n" .. stderr, { once = true, title = "Avante" })
+      end
+    end)
+
+    if not ok then
+      Utils.error("failed to run command: " .. cmd .. "\n" .. job_or_err)
+      return
+    end
+  else
+    value = os.getenv(key_name)
+  end
+
+  if value ~= nil then M.cache[cache_key] = value end
+
+  return value
+end
+
+return M

lua/avante/utils/init.lua

@@ -9,6 +9,7 @@ local lsp = vim.lsp
 ---@field root avante.utils.root
 ---@field file avante.utils.file
 ---@field history avante.utils.history
+---@field environment avante.utils.environment
 local M = {}
 
 setmetatable(M, {