Sync beefy commitment on demand

[vgeddes]

Syncs beefy commitment immediately if paraNonce > ethNonce, as long as the configured rate-limit is not exceeded.

The rate-limiting algorithm is implemented using a token bucket approach. For example, every hour, 1 token is added to the bucket. Every time a new commitment needs to be a synced, a token is consumed from the bucket as long as there are tokens available.

Algorithm:

while True
    Sleep(1min)
    ParaNonce, EthNonce := GetNonces()
    if ParaNonce > EthNonce:
        if !TokenBucket.TryConsume(1):
            Continue
        SyncLatestBeefyCommitment()
        WaitUntil {
            _, EthNonce := GetNonces() 
            EthNonce >= ParaNonce
        }

Can run this on-demand relayer using:

snowbridge-relay run beefy --on-demand ...

Added new configuration fields, mostly for rate-limiting configuration:

type OnDemandSyncConfig struct {
	// ID of the AssetHub channel
	AssetHubChannelID string `mapstructure:"asset-hub-channel-id"`
	// Maximum number of tokens available to consume
	MaxTokens uint64 `mapstructure:"max-tokens"`
	// Number of tokens added each `RefillPeriod`
	RefillAmount uint64 `mapstructure:"refill-amount"`
	// Period between token refills
	RefillPeriod uint64 `mapstructure:"refill-period"`
}

Infra companion: https://github.com/Snowfork/snowbridge-infra-suite/pull/28