Add Agent Skills support to the Common AI provider (#67786)

Add AgentSkillsToolset, a pydantic-ai toolset that loads agentskills.io SKILL.md bundles from a local directory or a Git repository. Git credentials come from an Airflow git connection (HTTPS token or SSH key) resolved through the Git provider's GitHook: cleartext http and credential-bearing URLs are rejected, interactive credential prompts are disabled, and the token is stripped from the clone's .git/config. Sources are resolved on the worker when the agent enters the toolset, so a token is never baked into the serialized DAG, and clones are removed when the run ends. Pass it via AgentOperator's toolsets=, or use it with a raw pydantic-ai Agent. The framework-agnostic resolve_skills() helper returns local SKILL.md directories for other Agent Skills loaders (LangChain DeepAgents, Strands).