Add graphql schema definition for balances query

[aditya1702]

What

• Add the graphql schema to support account balance queries

The following type of query will be supported:

balancesByAccountAddress(address: $address) {
				balance
				tokenId
				tokenType

				... on TrustlineBalance {
					code
					issuer
					type
					limit
					buyingLiabilities
					sellingLiabilities
					lastModifiedLedger
					isAuthorized
					isAuthorizedToMaintainLiabilities
				}
				... on SACBalance {
					code
					issuer
					decimals
					isAuthorized
					isClawbackEnabled
				}
				... on SEP41Balance {
					name
					symbol
					decimals
				}
			}

Why

Part of the account balances milestone: https://github.com/stellar/wallet-backend/milestone/13

Known limitations

N/A

Issue that this PR addresses

Closes #361

[aristidesstaffieri]

lgtm but looks like there is a go check warning in generated.go