feat(fx): add Transient provider option for non-cached constructors

[rezamokaram]

Motivation

Currently, fx.Provide() always registers constructors as singletons, meaning their results are reused for the lifetime of the app. In some cases, it’s desirable to have fresh instances, for example:

• Creating a new per-request context object
• Generating unique IDs or temporary objects
• Managing short-lived resources that shouldn’t be shared globally

Introduces a new fx.Transient() helper that allows constructors to be invoked each time their dependency is requested, rather than once at application start.

This enables transient or per-request dependency lifetimes similar to scoped services in other DI systems. The Transient wrapper registers a factory function that produces a new instance on each call.

Includes unit tests verifying:

• Constructor runs multiple times for transient providers.
• Singleton behavior remains unchanged for regular fx.Provide.

Backward compatibility

No breaking changes — existing Fx applications continue to behave identically. The new feature is purely additive.

Documentation and usage examples will be added in a follow-up PR after this change is approved.

[CLAassistant]

All committers have signed the CLA.

[rezamokaram]

Hi @sywhang, just a friendly ping on my PR (#1280) — whenever you have time to review it would be great. Thanks a lot!