Add wrappers to rpcclient to simplify for common ledger entry fetches

At minimum, introduce

- [ ] `getAccountEntry` 
- [ ] `getTrustline`
- [ ] `getClaimableBalance`

You can use https://github.com/stellar/js-stellar-sdk/pull/1221 and https://github.com/stellar/js-stellar-sdk/pull/1218 as references.