rpc: preserve global xpubs and proprietary fields in joinpsbts #35516

joinpsbts collects the global xpubs of all the joined PSBTs into merged_psbt, but returns a separately constructed shuffled_psbt into which only the inputs, outputs, and unknown fields are copied. The collected PSBT_GLOBAL_XPUB records are silently dropped, and PSBT_GLOBAL_PROPRIETARY records are not collected at all.

The xpub collection was added in #17034, which was written against a joinpsbts that still returned merged_psbt, but was merged after #16512 had introduced the shuffled_psbt rebuild, so the collected xpubs have never reached the result.

Shuffle the inputs and outputs of merged_psbt in place instead of rebuilding a new PSBT, so that all global data is preserved, and union the global proprietary records in the merge loop, matching the combinepsbt behavior from #34893.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35516.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35100 (rpc: Merge joinpsbts locktimes correctly by nervana21)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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cc @achow101 @w0xlt @theStack

Concept ACK