Config for Copilot OAuth Token

[giddie]

I wanted a little more control over the way I store the OAuth token for Copilot, and ended up landing on an additional oauth_token field for the adapter.

The diff is pretty simple -- see the commit here.

Here's an example that implements manual entry of the key:

require("codecompanion").setup({
  adapters = {
    copilot = function()
      oauth_token = vim.fn.inputsecret("Copilot OAuth Token: ")
      return require("codecompanion.adapters").extend("copilot", {
        env = {
          oauth_token = oauth_token
        }
      })
    end,
  }
})

Would you like a PR, or is there a different approach you'd like to take?