XChainBridge: Introduce sidechain support (XLS-38): (#4292)

A bridge connects two blockchains: a locking chain and an issuing
chain (also called a mainchain and a sidechain). Both are independent
ledgers, with their own validators and potentially their own custom
transactions. Importantly, there is a way to move assets from the
locking chain to the issuing chain and a way to return those assets from
the issuing chain back to the locking chain: the bridge. This key
operation is called a cross-chain transfer. A cross-chain transfer is
not a single transaction. It happens on two chains, requires multiple
transactions, and involves an additional server type called a "witness".

A bridge does not exchange assets between two ledgers. Instead, it locks
assets on one ledger (the "locking chain") and represents those assets
with wrapped assets on another chain (the "issuing chain"). A good model
to keep in mind is a box with an infinite supply of wrapped assets.
Putting an asset from the locking chain into the box will release a
wrapped asset onto the issuing chain. Putting a wrapped asset from the
issuing chain back into the box will release one of the existing locking
chain assets back onto the locking chain. There is no other way to get
assets into or out of the box. Note that there is no way for the box to
"run out of" wrapped assets - it has an infinite supply.

Co-authored-by: Gregory Popovitch <greg7mdp@gmail.com>

src/test/jtx/impl/attester.cpp

@@ -0,0 +1,82 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2022 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include <test/jtx/attester.h>
+
+#include <ripple/basics/Slice.h>
+#include <ripple/protocol/PublicKey.h>
+#include <ripple/protocol/STXChainBridge.h>
+#include <ripple/protocol/SecretKey.h>
+#include <ripple/protocol/XChainAttestations.h>
+
+namespace ripple {
+namespace test {
+namespace jtx {
+
+Buffer
+sign_claim_attestation(
+    PublicKey const& pk,
+    SecretKey const& sk,
+    STXChainBridge const& bridge,
+    AccountID const& sendingAccount,
+    STAmount const& sendingAmount,
+    AccountID const& rewardAccount,
+    bool wasLockingChainSend,
+    std::uint64_t claimID,
+    std::optional<AccountID> const& dst)
+{
+    auto const toSign = Attestations::AttestationClaim::message(
+        bridge,
+        sendingAccount,
+        sendingAmount,
+        rewardAccount,
+        wasLockingChainSend,
+        claimID,
+        dst);
+    return sign(pk, sk, makeSlice(toSign));
+}
+
+Buffer
+sign_create_account_attestation(
+    PublicKey const& pk,
+    SecretKey const& sk,
+    STXChainBridge const& bridge,
+    AccountID const& sendingAccount,
+    STAmount const& sendingAmount,
+    STAmount const& rewardAmount,
+    AccountID const& rewardAccount,
+    bool wasLockingChainSend,
+    std::uint64_t createCount,
+    AccountID const& dst)
+{
+    auto const toSign = Attestations::AttestationCreateAccount::message(
+        bridge,
+        sendingAccount,
+        sendingAmount,
+        rewardAmount,
+        rewardAccount,
+        wasLockingChainSend,
+        createCount,
+        dst);
+    return sign(pk, sk, makeSlice(toSign));
+}
+
+}  // namespace jtx
+}  // namespace test
+}  // namespace ripple