Add a prop to control playback speed

[SupersonicMagpie]

Currently it's only possible to specify a playback speed at edit time in the Rive editor. That's perfectly fine for normal animations, but in case of games it's sometimes required to dynamically change playback speed at the runtime.

[matt-aitken]

Definitely not just useful for games. We're building a marketing page and we want the animations to play slowly unless the animation is hovered.

[alisalahio]

+1 We need it at Instatus too!

Currently we need to create multiple animations with different speeds which is not efficient

[kj4483]

Similarly, music-related applications where tempo is important need this feature.