Implement scrub() with per-map field filtering

Key type bytes overlap across PSBT map types. For example, 0x06 is TX_MODIFIABLE in
global but BIP32_DERIVATION in inputs. So filtering requires map context.

Buffers the global map to detect version and counts (INPUT_COUNT/
OUTPUT_COUNT for v2, UNSIGNED_TX parsed via Transaction::consensus_decode
for v0), then streams remaining maps with per-map allowlists for global,
input, and output.

Pair::decode is pub(crate) in psbt-v2 0.3.0; worked around with a
PairDecode extension trait replicating the upstream logic.

Author: arminsabouri

Cargo.lock

@@ -9,4 +9,7 @@ workspace = true
 [features]
 default = ["unit-tests", "prop-tests"]
 unit-tests = []
-prop-tests = []
+prop-tests = []
+
+[dependencies]
+psbt-v2 = "0.3.0"

crates/scrubber/Cargo.toml

@@ -1 +1,4 @@
 #![forbid(unsafe_code)]
+
+pub mod scrub;
+pub use scrub::{Error, scrub};