[EPIC] generic tx building and signing #1180

### Critical for asset issuers to support advanced signing
- [x] Modifying existing commands so they can build a tx only: #1265

### Break up of currently unified behaviors to support composability
- [x] Add a `soroban tx simulate` command. That simulates any given tx and returns the modified/assembled tx that has fees, footprint, etc.
- [x] Add a `soroban tx send` command. Submits to the network, does nothing else.
- [x] Add a `soroban tx sign` command.That supports all existing and future signing capabilities just takes a tx base64 as input and returns it as output. https://github.com/stellar/stellar-cli/issues/1490

### Convenience commands for common tx XDR ops
- [x] Add a `soroban tx decode` command. This could be a lightweight wrapper around the XDR>JSON decoder command, `soroban lab xdr decode --type TransactionEnvelope`.
- [x] Add a `soroban tx encode` command. This could be a lightweight wrapper around the JSON>XDR encoder command, `soroban lab xdr encode --type TransactionEnvelope`.

### Bigger and more involved features
- [ ] Live edit experience in a `soroban tx edit` command. [Design doc](https://docs.google.com/document/d/1MHjQ-R6Dh_Ykhq5VcOT-d1-z5yoNrPC5RjNA0MEmEno/edit#heading=h.dn3a9fxqk3pq) Opened separate issue here: https://github.com/stellar/soroban-cli/issues/1290