This change introduces `GainProcessor` to apply gain automation over an
audio stream. The gain automation is defined with a `GainProvider`.
`DefaultGainProvider` implements a basic `GainProvider` that allows
populating a gain automation line additively by adding fade shapes
(`FadeProvider`) at specific positions with specific durations.
`DefaultGainProvider` also implements basic equal gain and equal power
fades.
The current `DefaultGainProvider` implementation does not support adding
fades at relative offsets from the stream end. Users must know the
stream duration in advance and add fades at absolute positions.
PiperOrigin-RevId: 733311314
c3d734066d