feat: add multisig sdk

[voliva]

I'm thinking I did a mistake adding the meta-signers to the top-level package. Back then it made sense since it was a new category of "meta-signers", so maybe having them next to the other signers would be nice.

But they work on a higher level, and the logic they do could be beneficial for things that are not signers.

The new idea in this PR is to have it as a standalone papi-sdk: it requires a typedApi with multisig stuff, and then exports two functions:

function getMultisigTx(
    multisig: MultisigAccount<Addr>,
    signatory: Addr,
    tx: Transaction,
    options?: MultisigTxOptions<Addr>,
): AsyncTransaction

function getMultisigSigner(
    multisig: MultisigAccount<Addr>,
    signer: PolkadotSigner,
    options?: MultisigTxOptions<Addr>,
): PolkadotSigner

They both need the multisig account info (signatories + threshold). Then for the multisigTx I need a signatory (since the call data changes based on who will sign it) and any Transaction (could do with call data only... but this way I can .getPaymentInfo() to get the weight... and you can always create one with typedApi.txFromCallData)

And getMulsitigSigner replaces the top-level one. Maybe we should deprecate that one and ask people to use this one instead?

[voliva]

I changed the tx parameter to txOrCallData, as I think it can be useful to pass in either of them, and the sdk can still just convert from one to the other.

[josepot]

Nice!