Combine Horizon and Soroban JS SDKs into a single repository

[Shaptic]

As part of the march to testnet, we should unify the stellar/js-soroban-client and stellar/js-stellar-sdk repositories as follows:

• Both will share the latest stellar-base version for Soroban (as of this writing, this is v10.0.0-soroban.4)
• RPC code will live under the SorobanRPC namespace
• Horizon code will live under the new Horizon namespace
• Shared code (e.g. TransactionBuilder and friends) will be merged and live together

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Previously, users would have the following scripts for separate Horizon / Soroban usage. Note that it's perfectly fine to synchronously submit a Soroban-shaped transaction to Horizon's POST /tx, you will just get a different response schema.

import StellarSdk from 'stellar-sdk';
import * as SorobanClient from 'soroban-client';

const horizon = StellarSdk.Server('https://horizon.stellar.org');
const rpc = SorobanClient.Server('https://rpc-futurenet.stellar.org');

const hzTx = StellarSdk.TransactionBuilder(...).build();
const soroTx = SorobanClient.TransactionBuilder(...).build();

horizon.submitTransaction(hzTx);
rpc.sendTransaction(soroTx);

Now, we will have

import { Horizon, SorobanRPC, TransactionBuilder } from 'stellar-sdk';

const horizon = Horizon.Server('https://horizon.stellar.org');
const rpc = SorobanRPC.Server('https://rpc-futurenet.stellar.org');

const anyTx = TransactionBuilder(...).build();

// two ways to submit, but **different responses**
horizon.submitTransaction(anyTx);
rpc.sendTransaction(anyTx);

Alternatively, they can do

import StellarSdk from 'stellar-sdk';

const horizon = StellarSdk.Horizon.Server('https://horizon.stellar.org');
const rpc = StellarSdk.SorobanRPC.Server('https://rpc-futurenet.stellar.org');

const anyTx = StellarSdk.TransactionBuilder(...).build();
// etc...

if they insist on using the default import method.