[SB-1967] GEMA smoothing

[SupernaviX]

Implement GEMA smoothing, where increases in the current price are "smoothed" based on the previous price. The window we're using is 10 rounds, we can change that in configuration later.

The output of the current round of signature aggregation depends on the output of older rounds. This required some other changes:

• The leader now sends the latest signed feed for each synthetic to its followers, even if that feed is from an older round. This helps newly joined/restarted nodes get in sync. We still only push "new" feeds to Butane, but the /payload endpoint includes the latest feed for everything.
• We do all price calculations in BigRationals (infinite-precision rational numbers). Price sources such as sundaeswap and bybit still report prices with Decimals (which have finite precision), but we convert that to BigRational before doing the math.
• Because BigRational can get arbitrarily large after many cycles of GEMA, we round prices in the final payload to take at most 1024 bits (128 bytes). This should be more than enough precision for what we need. We also round the older prices used for the EMA calculation to 512 bits (64 bytes).