Add a SorobanRpc.Server.getWasm(contractId) method

[Shaptic]

Is your feature request related to a problem? Please describe.
Building ledger entries by hand sucks and you need to understand too much about Soroban internals.

Describe the solution you'd like
See title, add a method to the server:

class Server {
  // ...
  getWasm(contractId: string): Promise<Buffer>;
}

which will make the correct underlying getLedgerEntries call for you.

Describe alternatives you've considered
Doing it manually:

const contractLedgerKey = new Contract(contractId).getFootprint();
const response = await server.getLedgerEntries(contractLedgerKey);
const wasmHash = response.entries[0].val.contractData().val().contractInstance().executable().wasmHash();
const ledgerKeyWasmHash = xdr.LedgerKey.contractCode(new xdr.LedgerKeyContractCode({ hash: wasmHash }));
const responseWasm = await server.getLedgerEntries(ledgerKeyWasmHash);
return responseWasm.entries[0].val.contractCode().code();

This is gross.

Additional context
See stellar-sdk#960, a review comment from @willemneal.

This is documented in the API reference for getLedgerEntries, but it'd still be better to make it built in.

[Shaptic]

Closed by #960