refactor: extract descriptor ID calculation from spkm GetID()

This allows us to verify the descriptor ID on the descriptors
unit tests in different software versions without requiring to
use the entire DescriptorScriptPubKeyMan machinery.

Note:
The unit test changes are introduced after the bugfix commit
but this commit + the unit test commit can be cherry-picked
on top of the v25 branch to verify IDs correctness. IDs must
be the same for v25 and after the bugfix commit.

src/script/descriptor.cpp

@@ -1778,6 +1778,14 @@ std::unique_ptr<Descriptor> InferDescriptor(const CScript& script, const Signing
     return InferScript(script, ParseScriptContext::TOP, provider);
 }
 
+uint256 DescriptorID(const Descriptor& desc)
+{
+    std::string desc_str = desc.ToString();
+    uint256 id;
+    CSHA256().Write((unsigned char*)desc_str.data(), desc_str.size()).Finalize(id.begin());
+    return id;
+}
+
 void DescriptorCache::CacheParentExtPubKey(uint32_t key_exp_pos, const CExtPubKey& xpub)
 {
     m_parent_xpubs[key_exp_pos] = xpub;