Add mimir wallet to lastic

[jarvandev]

We want to integrate mimir sdk in lastic, but there are some inconveniences

Problem:

mimir is a multisig wallet, and mimir will always inject the multisig account, so, when dapp want to sign and send transaction, it will wrapped by AsMulti() in mimir.

For Example, if you want to send balance.transfer, mimir will wrap this extrinsic to asMulti(balance.transfer(...args))

In lastic-sdk, the function signedTx cannot complete the problem described above.