Add config support for custom headers in Oauth2 access token request

[dragonhunt02]

I wrote a PowAssent custom Oauth2 strategy and controller for a native desktop app, but I ran into auth errors at

assent/lib/assent/strategies/oauth2.ex

Lines 280 to 288 in cd45572

	"""
	@spec grant_access_token(Keyword.t(), binary(), Keyword.t()) :: {:ok, map()} | {:error, term()}
	def grant_access_token(config, grant_type, params) do
	auth_method = Keyword.get(config, :auth_method)
	token_url = Keyword.get(config, :token_url, "/oauth/token")
	with {:ok, base_url} <- Assent.fetch_config(config, :base_url),
	{:ok, auth_headers, auth_body} <- authentication_params(auth_method, config) do
	headers = [{"content-type", "application/x-www-form-urlencoded"}] ++ auth_headers

The third-party service I use requires "X-Api-Version" header on access token requests.
I tried setting headers in strategy, but it doesn't seem to work.

I propose adding an optional config parameter to allow custom headers in Oauth2 request