Remove Assembled transaction

[leighmcculloch]

What

Remove Assembled transaction and simulate and assemble functionality from the client.

Why

Moving the logic into the stellar-cli.

It used to be part of the CLI and when the RPC client was separated from the CLI repo, the assembled tx type and it's related logic got pulled over into the CLI, but it's wrapped up in CLI logic.

The assembly logic would be useful to live somewhere else on its own, or as part of some sort of Rust Stellar SDK if one ever exists, but at the moment the boundary of that type is messy, and it's implementation has logging and other applications concerns wrapped up in it.

In the interest of focusing in the near term on the best CLI user experience, and not getting too caught up in "finding the ultimate code structure" for this which isn't a primary goal, moving it seems like the reasonable thing to do right now, so that through the course of evolving the CLI this type can evolve and find it's place and hopefully surface a better structure to separate into it's own "help you assemble txs" thing, or possibly back into the RPC if at that point it does less.

My hope is that by bringing it close to the CLI, who is the primary user, and unpacking and repacking it, we'll find an API and interface that is a sweet spot to either package on its own, or include back into the rpc client, and potentially use as a guide to implement similar elsewhere or potentially influence the api of the rpc. Likely some of this functionality will move into the xdr lib.

The Assembled type and related functionality are being added as much as possible as a simple copy-paste to limit unnecessary changes in:

• Add Assembled from stellar-rpc-client@9f74833 stellar-cli#1648

[willemneal]

Thanks! Definitely doesn't belong here.