cli: Support extracting sub-fields with jq-like queries

[leighmcculloch]

It would be really helpful if the stellar-xdr cli supported when decoding extracting subfields.

For example, the other day I was trying to get the memo extracted of a transaction, and I wanted to view the raw bytes of just that one field in the XDR. I was sort of able to do it with the following, but it was a pretty imperfect experience. I had to decode the entire tx, then re-encode the field I wanted to see in binary. I couldn't actually encode the field I wanted, the .text subfield of memo, because it has no named type. The best I could do was re-encode the memo field which wraps the text field.

$ curl -sSL https://horizon.stellar.org/transactions/1a0d08578d973892ef99e46c6a33882e231eb17a8c4f503487b996bb17982b57 \
    | jq -r '.envelope_xdr' \
    | stellar xdr decode --type TransactionEnvelope \
    | jq -r '.tx.tx.memo' \
    | stellar xdr encode --type Memo --output single \
    | xxd
00000000: 0000 0001 0000 000c d349 1c68 30d6 13e6  .........I.h0...
00000010: 28db 168d                                (...

The experience should be something like:

$ curl -sSL https://horizon.stellar.org/transactions/1a0d08578d973892ef99e46c6a33882e231eb17a8c4f503487b996bb17982b57 \
    | jq -r '.envelope_xdr' \
    | stellar xdr decode --type TransactionEnvelope --query '.tx.tx.memo.text' --output single-binary \
    | xxd
00000000: 0000 000c d349 1c68 30d6 13e6 28db 168d  .....I.h0...(...

Ref to offline conversation:

• https://stellarfoundation.slack.com/archives/C065T73DT60/p1745839811776739?thread_ts=1745837398.666689&cid=C065T73DT60

cc @stellar/devx

[leighmcculloch]

Cursory look at how we might do this suggests this might be a pretty big lift to the point that the effort might outsize the value.