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/usage_api.ex

@@ -0,0 +1,37 @@
+defmodule TelecomSDK.UsageAPI do
+  @moduledoc """
+  API for usage management
+  """
+
+  defstruct [:client]
+
+  def new(client) do
+    %__MODULE__{client: client}
+  end
+
+  def get_stats(api, subscriber_id, start_date, end_date) do
+    params = %{
+      start_date: DateTime.to_iso8601(start_date),
+      end_date: DateTime.to_iso8601(end_date)
+    }
+    TelecomSDK.HTTPClient.get(api.client, "/v1/subscribers/#{subscriber_id}/usage", params)
+  end
+
+  def list_events(api, opts \\ []) do
+    params = %{
+      page: Keyword.get(opts, :page, 1),
+      page_size: Keyword.get(opts, :page_size, 50)
+    }
+
+    params = if Keyword.get(opts, :subscriber_id), do: Map.put(params, :subscriber_id, Keyword.get(opts, :subscriber_id)), else: params
+    params = if Keyword.get(opts, :usage_type), do: Map.put(params, :usage_type, Keyword.get(opts, :usage_type)), else: params
+    params = if Keyword.get(opts, :start_date), do: Map.put(params, :start_date, DateTime.to_iso8601(Keyword.get(opts, :start_date))), else: params
+    params = if Keyword.get(opts, :end_date), do: Map.put(params, :end_date, DateTime.to_iso8601(Keyword.get(opts, :end_date))), else: params
+
+    TelecomSDK.HTTPClient.get(api.client, "/v1/usage/events", params)
+  end
+
+  def get_realtime(api, subscriber_id) do
+    TelecomSDK.HTTPClient.get(api.client, "/v1/subscribers/#{subscriber_id}/realtime")
+  end
+end