feat: add `tx::builder` module and initial classic commands

[willemneal]

What

Add a transaction builder and operation builder trait. Also adds the first two stellar classic commands under the subcommand tx new:

• [x] Create account
• [x] Payment
• [x] Set options
• [x] Bump sequence
• [x] Account Merge
• [x] Manage data
• [x] InvokeFunction
• [x] Extend footprint ttl
• [x] Restore footprint
• [x] Allow trust
• [x] Change trust
• [x] Set trustline flags
• [x] Doc review

Todo:

• [x] Tests

Why

Currently all transactions are built by hand and require a lot of knowledge around the internal types. Furthermore, there is a lot of redundant code around creating, submitting transaction.

These examples should make it easy to have a quick design discussion before adding the rest of the operations.

Known limitations

[TODO or N/A]