wallet: avoid returning a reference to vMasterKey after releasing the mutex that guards it

`CWallet::GetEncryptionKey()` would return a reference to the internal
`CWallet::vMasterKey`, guarded by `CWallet::cs_wallet`, which is unsafe.

Returning a copy would be a shorter solution, but could have security
implications of the master key remaining somewhere in the memory even
after `CWallet::Lock()` (the current calls to
`CWallet::GetEncryptionKey()` are safe, but that is not future proof).

So, instead of `EncryptSecret(m_storage.GetEncryptionKey(), ...)`
change the `GetEncryptionKey()` method to provide the encryption
key to a given callback:
`m_storage.WithEncryptionKey([](const CKeyingMaterial& k) { EncryptSecret(k, ...); })`

This silences the following (clang 18):

```
wallet/wallet.cpp:3520:12: error: returning variable 'vMasterKey' by reference requires holding mutex 'cs_wallet' [-Werror,-Wthread-safety-reference-return]
 3520 |     return vMasterKey;
      |            ^
```

1 files changed, 3 insertions, 2 deletions

diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index e03f5532fc..e93cd4b4b9 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -3513,9 +3513,10 @@ void CWallet::SetupLegacyScriptPubKeyMan()
     AddScriptPubKeyMan(id, std::move(spk_manager));
 }
 
-const CKeyingMaterial& CWallet::GetEncryptionKey() const
+bool CWallet::WithEncryptionKey(std::function<bool (const CKeyingMaterial&)> cb) const
 {
-    return vMasterKey;
+    LOCK(cs_wallet);
+    return cb(vMasterKey);
 }
 
 bool CWallet::HasEncryptionKeys() const