client: backup trade recovery data to Skynet web3portal

[norwnd]

This is a rough MVP of an idea outlined in decred#952 (comment) that can be used by tech-savvy DEX users.

It's vendor-specific(and prevents trades when vendor service is unavailable), has weak privacy, yet it should do the job. This implementation is currently using https://web3portal.com vendor that allows for up to 2500 free uploads, which should be sufficient for most use-cases.

However I haven't tested it for Taker redeem (not 100% sure if it'll work): redeem is problematic, if Maker already redeemed Taker will need secret + secretHash in order to redeem his part (he can't refund after this point)

Well now run a test for this, it does work for Taker redeem actually, just need to:

• find Maker redeem and fetch secret, for DCR might need to check FindKeyPush in dcrdex repo on how to extract secrets from DCR redeem txns, eg:

sigScript, err := hex.DecodeString("483045022100e619b9ecc2fd2ec4e3f38be09688c722661e8a1ef3dce09196a7f5dd30eaa25102202fa58fc4344f844ba3194a519c956f0c1baf8b82353d4b3cb9a8e5ba344e0b3901210380dcc0f97c89b186aab2329bae4b30ee920e17a3b6435e4c121178d908cdb4de20e365285e21fc311069cf206d9e56c8eaab996ae84d4c18787640cdb7a6c91253514c616382012088c020a017a12d2dc3f88f98c3225e226e16a5a1d3190a725ab90c32e5b6115620b7d48876a9148a00aafbd40a6e7b619a7b2ca0a8763b4b57507f6704520cbb64b17576a914ce45bc063873330be210f80ebd21acfd9aaf66226888ac")
	if err != nil {
		return fmt.Errorf("hex.DecodeString: %v", err)
	}
	contractHash, err := hex.DecodeString("b5c5936018751a6f427dfc524de0c8f547d183f4")
	if err != nil {
		return fmt.Errorf("hex.DecodeString: %v", err)
	}
	secret, err := FindKeyPush(0, sigScript, contractHash, chaincfg.TestNet3Params())
	if err != nil {
		return fmt.Errorf("hex.DecodeString: %v", err)
	}
	fmt.Println(fmt.Sprintf("secret hash: %x", sha256.Sum256(secret)))
	fmt.Println(fmt.Sprintf("secret: %x", secret))

• then use this secret with the following code to broadcast Taker redeem transaction (using Decred wallet in this example, can put this into (dcr *ExchangeWallet) Unlock after successful wallet connect&unlock, and one needs to login in UI to trigger this code to run):

	hash, err := chainhash.NewHashFromStr("1793f5eebbfae6332744c48fd79f73b18c352e6375763a43f4da67fe7a5ae700")
	if err != nil {
		panic(err)
	}
	if err != nil {
		return nil, fmt.Errorf("chainhash.NewHashFromStr: %w", err)
	}
	contract := dex.Bytes{}
	err = contract.UnmarshalJSON([]byte(`"6382012088c0204facf7cbc8b60d05a88712b6c2e5ee82d8cfd7d2bd6805175e31923e498675ae8876a91439572304d4b32fe54f64f484ec6f06f268ecc0ed6704005eb864b17576a91460271efb85cb69f2a95e0fa205130c811bce9b946888ac"`))
	if err != nil {
		return nil, fmt.Errorf("contract.UnmarshalJSON: %w", err)
	}
	swapTx := dex.Bytes{} // https://testnet.dcrdata.org/tx/1793f5eebbfae6332744c48fd79f73b18c352e6375763a43f4da67fe7a5ae700
	err = swapTx.UnmarshalJSON([]byte(`"01000000013ac2f0308badac05af17334552b05044284f7f666ecb3674b8e76f1869dddd8c0000000000ffffffff0100c2eb0b00000000000017a914507e89df445fc26d92faae65bd86634081c917c287000000000000000001bacfeb0b000000000e0b11000800000069463043021f01bbbd5a4856184724efcb025744e3a5ecae6b2699e61f1d84a36b821f502f0220665ec826f38b8044f9a351e6a952c06b98f8a45f83f1ef083debde7d7c1607c10121032e65ecca0563a73aea48d95e5730c1d9f05d53c400fdb652974331e7f14ba7ad"`))
	if err != nil {
		return nil, fmt.Errorf("swapTx.UnmarshalJSON: %w", err)
	}
	info, err := dcr.AuditContract(toCoinID(hash, 0), contract, swapTx,  false)
	if err != nil {
		return nil, fmt.Errorf("dcr.AuditContract: %w", err)
	}
	secret := dex.Bytes{}
	err = secret.UnmarshalJSON([]byte(`"a25899484e1dfc660a5d3c5a3ae00d19650a300c1f1357b19ceba90a2d87e08c"`))
	if err != nil {
		return nil, fmt.Errorf("secret.UnmarshalJSON: %w", err)
	}
	coinID, out, _, err := dcr.Redeem(
		&asset.RedeemForm{
			Redemptions:[]*asset.Redemption{
				{
					Spends: info,
					Secret: secret,
				},
			},
		},
	)
	if err != nil {
		return nil, fmt.Errorf("dcr.Redeem: %w", err)
	}
	dcr.log.Infof("GOT coinID, out: %s, %s", spew.Sdump(coinID), spew.Sdump(out))