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/graphql_api.ex

@@ -0,0 +1,17 @@
+defmodule TelecomSDK.GraphQLAPI do
+  @moduledoc """
+  API for GraphQL queries
+  """
+
+  defstruct [:client]
+
+  def new(client) do
+    %__MODULE__{client: client}
+  end
+
+  def execute(api, query, variables \\ nil) do
+    request = %{query: query}
+    request = if variables, do: Map.put(request, :variables, variables), else: request
+    TelecomSDK.HTTPClient.post(api.client, "/graphql", request)
+  end
+end