mempool: add reduced-data ignore-reject to relay grandfathered spends

[kwsantiago]

Follow-up to #322 (closed). The reduced-data relay rejection is intentional, but a miner should be able to opt into relaying/accepting spends of pre-activation ("grandfathered") UTXOs, which consensus already exempts in ConnectBlock.

Adds a "reduced-data" ignore_rejects token (usable via sendrawtransaction/testmempoolaccept). Default behavior is unchanged: those spends stay rejected at relay. When the token is set, inputs spending pre-activation UTXOs are exempted per-input in both mempool script checks (Policy and Consensus), matching ConnectBlock so the two checks stay consistent. Non-grandfathered spends stay rejected even with the token, since they are genuinely reduced-data-invalid.

Test: feature_reduced_data_utxo_height.py adds a case where the grandfathered OP_IF-tapscript spend is rejected by default, accepted with the token, and a non-grandfathered spend is still rejected with the token.

[pdath]

ctACK 0233128

The complexity of this change exceeds my knowledge, so I can not do a human code review.

I reviewed the code with Code Rabbit and it reported a single "nit" in "feature_reduced_data_utxo_height.py", lines 319-342:
"Assert the reject-reason prefix here
res["allowed"] alone doesn’t distinguish a reduced-data policy hit from an unrelated failure. Check reject-reason too: the grandfathered case should hit mempool-script-verify-flag-failed, and the non-grandfathered case should hit mandatory-script-verify-flag-failed."

I tested using the following commands on Ubuntu 24.04 running on ARMv8-A (64-bit):

git clone https://github.com/bitcoinknots/bitcoin.git pr324
cd pr324
git fetch origin pull/324/head:pr324
git switch pr324

cmake -B build
-DRDTS_CONSENT=IMPLICIT
-DBUILD_GUI=OFF
-DCMAKE_BUILD_TYPE=Release
nice cmake --build build -j4

cd build
ctest -R "validation_tests|mempool_tests" --output-on-failure
python3 test/functional/test_runner.py feature_rdts.py feature_reduced_data_utxo_height.py