Feature request: Add `Source.multi` method

Motivation:

There are many `Source.from(Arrays.asList(...))` in Java, I think we can add something like `Source.multi(...)` which work as the `Flux.Just(T ...t)`.

Result:
Replace `Source.from(Arrays.asList(1,2,3))` to `Source.multi(1,2,3))`

Locate the Source.from API and any existing Source tests, then compare their behavior with the requested Flux.Just-style varargs call. Done means Source.multi(1,2,3) provides the same result as Source.from(Arrays.asList(1,2,3)), with coverage for the new API.

Written by the indexing model from the issue text.