event: adding FeedOf[T]

[fjl]

Package event provides type Feed for subscription handling. A Feed is always constrained to a single type, and this restriction is enforced at runtime using reflection. With the introduction of generics in Go 1.18, it is now possible to declare and enforce the element type of Feed at compile time.

To avoid breaking downstream code, I suggest we add a new type FeedOf[T any] to package event. The behavior of this feed implementation would be exactly the same as the current one.

[fjl]

Note for potential contributors: this issue is not actionable immediately since we will wait for a while to fully adopt Go 1.18 and generics. Please do not submit PRs for this issue right now.