Add support for external signing to the SDK

[danielost]

Currently transaction builder performs tweaking and signing under the hood, e.g:

op_rand/crates/transaction-builder/src/transaction_builder.rs

Line 165 in dbeeb5e

.sign_psbt_input(&mut psbt, input_index + 1, *amount, None)?;

Decompose the functions in such a way that signing can be performed independently from the transaction builder