services: add missing type annotations

[Lotram]

Add return type and argument annotations across all services to satisfy the ANN ruff rules, which are now enforced on the full bugwarrior/ package.

• fixes a latent bug in NextcloudDeckService.get_owner, which treated a NextcloudDeckIssue as a subscriptable dict (TypeError at runtime when only_if_assigned was configured), and adds a regression test.
• Remove Client inheritance from any Service subclass. Since this Client class only has a single staticmethod, and nothing else, we call Client.json_responses directly instead. Services that subclass Client already use it an attribute rather than a parent class, so this change adds consistency between all services
• use explicit __init__ parameters for all Service subclasses (using config and main_config), as they are required anyway, for easier typing.
• gitlab and github use type aliases, to improve readability