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, 2 insertions, 1 deletions

diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index 487921443f..cc961068a5 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -962,7 +962,8 @@ public:
     //! Make a LegacyScriptPubKeyMan and set it for all types, internal, and external.
     void SetupLegacyScriptPubKeyMan();
 
-    const CKeyingMaterial& GetEncryptionKey() const override;
+    bool WithEncryptionKey(std::function<bool (const CKeyingMaterial&)> cb) const override;
+
     bool HasEncryptionKeys() const override;
 
     /** Get last block processed height */