[rpc]blindrawtransaction accepts more commitments

[AkioNak]

masterのblindrawtransactionはcommitmentの数とblind対象のtransactionのinputの数が一致する必要がある。
しかし例えばconfidential asset demo はpartial blinding を使用するが、
e14 では blindrawtransaction のcommitmentは最終的なinputの数を用意して引き渡していた。

このPRはe14と同様の動作に戻したい。

参考： ElementsProject#510 ElementsProject#550

[kallewoof]

In ElementsProject#510 / ElementsProject#550 a requirement that the blinding commitments were equal in amount to the inputs was introduced. This requirement is correct for the final case, but not for partial transactions (e.g. where party 1 blinds their inputs before even handing the transaction over to party 2 to add theirs). In order to hide party 1's input amounts from party 2, blindrawtransaction must allow >= the inputs, since the commitments are shared beforehand (if not, the blinding fails; this may need further investigating).

This PR restores the functionality to allow blinding commitments more than or equal to the amount of inputs, which makes the confidential assets demo https://github.com/ElementsProject/confidential-assets-demo functional again.