fix checkpoint taproot tree

[louisinger]

This PR includes two fixes to build_offchain_transactions.

TaprootTree encoding

Firstly, it fixes the taproot tree encoding by removing the extra write_compact_size_uint at the beginning of the serialization. That's something we introduced recently to be compliant with orignal taptree encoding: arkade-os/arkd#717 (the PR also contains a fixture to unit test it).

Add checkpoint_script to taproot tree

While encoding the scripts added to the off chain transaction as VTXO_TAPROOT_KEY unknown field, the checkpoint_script must also be included. That's something we need to verify server-side (once arkade-os/arkd#757 merged, it will fail if checkpoint_script is not included)

Please forgive my Rust code

@luckysori please review

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