chain: add SubmitPackage API for atomic parents+child relay

[bhandras]

Change Description

This commit introduces first-class support for Bitcoin Core's submitpackage RPC (v26+), allowing btcwallet users to broadcast (potentially zero‑fee) parent txns together with their (fee‑paying) child txn in one call.

Depends on: btcsuite/btcd#2366

Steps to Test

N/A

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[sputn1ck]

I think we shouldn't skip on Error. There is also a 'PackageMessage' field in btcjson.SubmitPackageResult which we could use, this can be "success" or "transaction failed" (maybe more). I think if "transaction failed" we'd want to get the error message of each tx. See:

SubmitPackage response: (btcjson.SubmitPackageResult) {
         PackageMsg: (string) (len=18) "transaction failed",
         TxResults: (map[string]btcjson.SubmitPackageTxResult) (len=2) {
          (string) (len=64) "a1946b6a4318e4cf598778d74dd4377a0417294473b2223751e7cc4ba52e93ec": (btcjson.SubmitPackageTxResult) {
           TxID: (string) (len=64) "b46359e30f5b8f5a5a961fedd225b44e264687f8a21c204a7cfd8ee0e305ee3e",
           OtherWtxid: (*string)(<nil>),
           VSize: (int64) 0,
           Fees: (btcjson.SubmitPackageFees) {
            Base: (float64) 0,
            EffectiveFeeRate: (*float64)(<nil>),
            EffectiveIncludes: ([]string) <nil>
           },
           Error: (*string)(0xc00128d580)((len=30) "bad-txns-inputs-missingorspent")
          },
          (string) (len=64) "4e080b762bc75de2e48b40b43eb36bd36b6faaa72bb99a3ae03b0b2f7e66629a": (btcjson.SubmitPackageTxResult) {
           TxID: (string) (len=64) "9bc86f989b423e4189413fcbbda4c7820edca5fcce4a8078333d4a91fecb5d48",
           OtherWtxid: (*string)(<nil>),
           VSize: (int64) 0,
           Fees: (btcjson.SubmitPackageFees) {
            Base: (float64) 0,
            EffectiveFeeRate: (*float64)(<nil>),
            EffectiveIncludes: ([]string) <nil>
           },
           Error: (*string)(0xc00128d5a0)((len=30) "bad-txns-inputs-missingorspent")
          }
         },
         ReplacedTransactions: ([]string) {
         }
        }

success:

 lnd_wallet.go:379: SubmitPackage response: (btcjson.SubmitPackageResult) {
         PackageMsg: (string) (len=7) "success",
         TxResults: (map[string]btcjson.SubmitPackageTxResult) (len=2) {
          (string) (len=64) "00063b73b1b564833e552e70ce3ce1ff2d6c101d5c44d0acc755c9553ff0be4b": (btcjson.SubmitPackageTxResult) {
           TxID: (string) (len=64) "4facdd76aa1d4dbc6e0324b0df4eb398b261a51649831db43b20bc266270cafb",
           OtherWtxid: (*string)(<nil>),
           VSize: (int64) 110,
           Fees: (btcjson.SubmitPackageFees) {
            Base: (float64) 0,
            EffectiveFeeRate: (*float64)(0xc000d42e00)(0),
            EffectiveIncludes: ([]string) (len=1 cap=1) {
             (string) (len=64) "00063b73b1b564833e552e70ce3ce1ff2d6c101d5c44d0acc755c9553ff0be4b"
            }
           },
           Error: (*string)(<nil>)
          },
          (string) (len=64) "0dfbd5cf4dd7f280afa79c3bcb2b919876eec1237273f65ad6761a61d5e88f20": (btcjson.SubmitPackageTxResult) {
           TxID: (string) (len=64) "4091d93ab8e0ef79d646c930bd0a4a8cb103b243b3f1b5b31143e33f8daa171e",
           OtherWtxid: (*string)(<nil>),
           VSize: (int64) 110,
           Fees: (btcjson.SubmitPackageFees) {
            Base: (float64) 1.097e-05,
            EffectiveFeeRate: (*float64)(0xc000d42e08)(9.972e-05),
            EffectiveIncludes: ([]string) (len=1 cap=1) {
             (string) (len=64) "0dfbd5cf4dd7f280afa79c3bcb2b919876eec1237273f65ad6761a61d5e88f20"
            }
           },
           Error: (*string)(<nil>)
          }
         },
         ReplacedTransactions: ([]string) {
         }
        }

[bhandras]

Agree with you, the initial implementation was created more for quickly testing the feature, but now added a higher level result type to btcjson so we get back more data. PTAL.