Smartsig signer within the dApp

## Problem

AVM is capable of handling inner transactions. The only signer which is currently available is the current application signer which can sign inner app calls, payment tx, asset transaction but only from the application account or the account rekeyed to the application account.

Please add support for the smart sig signer.

We are building dApp which depends on the specifications of the other projects. The goal is to deploy smart contract application which takes some deposit and combines multiple ecosystem projects application and performs actions as it would be done by the SDK. The issue with some applications is that it requires smart sigs as part of the grouped transactions. For example AlgoFi V2 protocol requires signature of this account in order to allow landing: https://algoexplorer.io/address/ITG4MVMAQXLVEWLUH5KGRHSPVQUVHDTOCHN2WD2NZXZVZWWXHSJPAX4UEU

There are dozens of other examples, for example AlgoDex, ARC0017, ... If developers could combine freely different algorand protocols, it would be great boost for the ecosystem and DeFi growth.

## Solution

Please create new opcode such as ```itxn_sig``` which would allow developers to specify the signature program.

## Dependencies

No

## Urgency

I expect this feature to be available within 6 to 12 months.