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/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, {