[ecash-lib] Add a fromTx method to the TxBuilder

Summary:
In order for postage to work, we need a way of sending a serialized partially-signed tx to a server. The server then must recreate a TxBuilder from this partially signed tx in order to add fuel inputs.

Add and test a method to support this.

Ref D18837 to see where this is going

Test Plan: npm test

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D18838