docs: Add Elixir SDK README with comprehensive API documentation and implement modular architecture with GenServer support

- Add README.md with installation instructions, architecture overview, and API examples for all SDK modules
- Refactor TelecomSDK main module to use modular API clients (SubscribersAPI, UsageAPI, PaymentAPI, RatingPlanAPI, SystemAPI, GraphQLAPI)
- Add AuthProvider class with API key and JWT token support (generate_jwt_token, validate_jwt_token methods)
- Add HTTPClient base

sdk/elixir/lib/telecom_sdk/rating_plans_api.ex

@@ -0,0 +1,19 @@
+defmodule TelecomSDK.RatingPlanAPI do
+  @moduledoc """
+  API for rating plan management
+  """
+
+  defstruct [:client]
+
+  def new(client) do
+    %__MODULE__{client: client}
+  end
+
+  def list(api) do
+    TelecomSDK.HTTPClient.get(api.client, "/v1/rating-plans")
+  end
+
+  def get(api, plan_id) do
+    TelecomSDK.HTTPClient.get(api.client, "/v1/rating-plans/#{plan_id}")
+  end
+end