perf: lazily import the interactions API to speed up import google.genai

The top-level package eagerly imported the interactions submodule, and
client.py eagerly imported the _gaos backend, so every `import
google.genai` paid for hundreds of interactions-API modules even when the
caller only uses generate_content.

Defer both: make `interactions` a lazy module attribute via __getattr__,
and move the _gaos imports in client.py under TYPE_CHECKING plus into the
Client/AsyncClient properties that construct them. `from __future__ import
annotations` keeps the property return annotations valid without the eager
import. The _gaos modules now load only when .interactions/.agents/
.webhooks (or the nextgen client) are first accessed.

PiperOrigin-RevId: 963526443