Add peer liquidity high and asset liquidity high events

[mkurapov]

Context

Similar to how we have peer.liquidity_low and asset.liquidity_low webhook events, we should add a peer.liquidity_high and asset.liquidity_high webhook events.

Rafiki's A peer liquidity increases whenever payments are being sent from it, which, in other words, means the debt owing to the other peer increases. This information would be useful for the ASE to know as soon as it happens, to better manage their risk, and liquidity in general.

When it comes to asset liquidity, most of the changes to asset liquidity happen during cross-currency transfers (as it's necessary to convert from one asset account to another). Naturally, asset low and asset high liquidity events will go hand in hand (ie if sending between USD <> EUR, USD will decrease, EUR will increase, Rafiki could end up firing the asset low and asset high events), but this also seems useful to have from a FX risk perspective.

Todos

• Add optional liquidityHighThreshold input variable to createPeer createAsset updatePeer updateAsset graphql mutations, and store this in the asset and peer tables
• Add peer.liquidity_high and asset.liquidity_high webhook events. These events should get fired in the onCredit methods for assets & peers.
• Create documentation issue for this