Add Bulk Transfer Ability

We'd like the ability to "bulk" transactions - if you have STX spread across many wallets, it'd be very useful to avoid individual transfers for each of those.

The main ask is that we need the ability to specify to transfer XX,XXX STX from each of a list of wallets, to a new target wallet to combine them all into.

Additionally, since these are BTC transactions, it would be great to be able to bulk fund all of the underlying BTC wallets with the appropriate amount of BTC for the transactions (though as long as estimations are good this could also be done via a normal btc wallet).

Start by reading the existing JavaScript webscript for generating, signing, and decoding multisig STX operations in the STX-over-BTC format. Identify how individual transfers and their underlying BTC funding are handled; done means supporting transfers from a list of wallets to one target wallet, with appropriate BTC funding or reliable estimates.

Written by the indexing model from the issue text.