Merge bitcoin/bitcoin#30048: crypto: add NUMS_H const

9408a04e42 tests, fuzz: use new NUMS_H const (josibake)
b946f8a4c5 crypto: add NUMS_H const (josibake)

Pull request description:

  Broken out from #28122

  ---

  [BIP341](https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki#constructing-and-spending-taproot-outputs) defines a NUMS point `H` as *H = lift_x(0x50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0)* which is [constructed](11af7015de/src/modules/rangeproof/main_impl.h (L16)) by taking the hash of the standard uncompressed encoding of the [secp256k1](https://www.secg.org/sec2-v2.pdf) base point G as X coordinate."

  Add this as a constant so it can be used in our codebase. My primary motivation is BIP352 specifies a special case for when taproot spends use `H` as the internal key, but outside of BIP352 it seems generally useful to have `H` in the codebase, for testing or other use cases.

ACKs for top commit:
  paplorinc:
    re-ACK 9408a04e42
  achow101:
    ACK 9408a04e42
  theStack:
    Code-review ACK 9408a04e42

Tree-SHA512: ad84492f5d635c0cb05bd82546079ded7e5138e95361f20d8285a9ad6e69c10ee2cc3fe46e16b46ef03c4253c8bee1051911c6b91264c90c3b1ad33a824bff4b

src/test/fuzz/miniscript.cpp

@@ -309,9 +309,6 @@ const struct KeyComparator {
 // A dummy scriptsig to pass to VerifyScript (we always use Segwit v0).
 const CScript DUMMY_SCRIPTSIG;
 
-//! Public key to be used as internal key for dummy Taproot spends.
-const std::vector<unsigned char> NUMS_PK{ParseHex("50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0")};
-
 //! Construct a miniscript node as a shared_ptr.
 template<typename... Args> NodeRef MakeNodeRef(Args&&... args) {
     return miniscript::MakeNodeRef<CPubKey>(miniscript::internal::NoDupCheck{}, std::forward<Args>(args)...);
@@ -1018,7 +1015,7 @@ CScript ScriptPubKey(MsCtx ctx, const CScript& script, TaprootBuilder& builder)
 
     // For Taproot outputs we always use a tree with a single script and a dummy internal key.
     builder.Add(0, script, TAPROOT_LEAF_TAPSCRIPT);
-    builder.Finalize(XOnlyPubKey{NUMS_PK});
+    builder.Finalize(XOnlyPubKey::NUMS_H);
     return GetScriptForDestination(builder.GetOutput());
 }