rpc: Enhance metadata of the dumptxoutset output

Summary:
The following data is added:

• A newly introduced utxo set magic
• A version number
• The network magic
• The block height

This concludes backport of core#29612
https://github.com/bitcoin/bitcoin/pull/29612/commits/542e13b2937356810bda2c41be83c3b1675e2f2f

Depends on D17989

Test Plan:
ninja all check-all

Dump an assumeutxo snapshot on a fully synced node, load it on a fresh new node, let it fully validate the chain

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17994