Subgraph mapping for Kleros registry recipient address and metadata

[yuetloo]

This issue was introduced by PR (#473).

The issue is to add mapping rule for recipientAddress and recipientMetadata In KlerosRecipientRegistryMapping.ts, the function handleRecipientAdded(event: RecipientAdded). Currently, they are commented out. The challenge is to parse the metadata field and to retrieve data from external contract, the TCR contract, see https://kleros.io/curation/ for more details.

Because the UI currently retrieves the recipient information from the contract directly (not from subgraph), I'll leave this issue to be tackled at another time to keep the scope of the original PR small.

This is how the UI currently gets the recipient information from the contract.
https://github.com/clrfund/monorepo/blob/develop/vue-app/src/api/recipient-registry-kleros.ts#L70