feat: Restore instance-level plugins

[jterapin]

Context

Restores V3 behavior lost in the V4 rewrite:

client = Service::Client.new(plugins: [MyPlugin])  # scoped to this client

• Single-pass only; plugins can't inject other plugins (deliberate vs V3).
• String-name fix — require_plugin resolves constants from the global namespace (Kernel) instead of Smithy::Client::Plugin, so a string-named plugin colliding with a constant under Plugin no longer mis-resolves.

Other Notes

• De-dup matches V3 (by object identity): a class vs. its instance, and two instances of one class, are not deduped. Per-class uniqueness is a separate discussion, out of scope here. I made a TODO for a visit later with the team.