services/horizon: Smart Contracts: Plan work for horizon support.

[paulbellamy]

Once either https://github.com/stellar/stellar-protocol/blob/master/core/cap-0050.md or https://github.com/stellar/stellar-protocol/blob/master/core/cap-0052.md lands. Let's get started, the caps will be finalized after, but in the meantime we can ship an alpha to help the community get started building. For now this work can all be done on a branch (soroban would match other repos), and we can tell people how to build from there, or we can build a special package.

TODO

• Understand the impact of these changes from the caps. There's a few caps. so do each.
• services/soroban-rpc: Support new XDR #4553
  • see: https://github.com/stellar/stellar-xdr-next
  • txn envelope and operation for ingestion
  • new ledger close metas, including SCVal types and events
• services/soroban-rpc: getEvents method #4554
• services/soroban-rpc: simulateTransaction, sendTransaction, and getTransactionStatus methods #4555
  • Depends on Smart Contracts: Current-state RPC Node for contract querying stellar-core#3484
• services/soroban-rpc: getAccount method v1 (sequence only) #4556
  • Maybe each balance should be a pair (classic/smart)?
• services/soroban-rpc: Write API docs and getting started guide stellar-cli#451

[paulbellamy]

Related js sdk: stellar/js-stellar-sdk#787