Add py.typed marker file for mypy

[MHHukiewitz]

From running mypy on aleph-sdk-python:

src/aleph/sdk/client/http.py:8:1: error: Skipping analyzing "aleph_message": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/aleph/sdk/client/http.py:9:1: error: Skipping analyzing "aleph_message.models": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/aleph/sdk/client/authenticated_http.py:10:1: error: Skipping analyzing "aleph_message": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/aleph/sdk/client/authenticated_http.py:11:1: error: Skipping analyzing "aleph_message.models": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/aleph/sdk/client/authenticated_http.py:29:1: error: Skipping analyzing "aleph_message.models.execution.base": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/aleph/sdk/client/authenticated_http.py:30:1: error: Skipping analyzing "aleph_message.models.execution.environment": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/aleph/sdk/client/authenticated_http.py:35:1: error: Skipping analyzing "aleph_message.models.execution.instance": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/aleph/sdk/client/authenticated_http.py:36:1: error: Skipping analyzing "aleph_message.models.execution.program": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/aleph/sdk/client/authenticated_http.py:37:1: error: Skipping analyzing "aleph_message.models.execution.volume": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/aleph/sdk/client/authenticated_http.py:38:1: error: Skipping analyzing "aleph_message.status": module is installed, but missing library stubs or py.typed marker  [import-untyped]