Merge bitcoin/bitcoin#34741: refactor: Return std::optional from GetNameProxy/GetProxy

fa73ed467c refactor: Fix redundant conversion to std::string and then to std::string_view [performance-string-view-conversions] (MarcoFalke)
fa270fdacf refactor: Return std::optional from GetProxy (MarcoFalke)
faeac1a931 refactor: Return std::optional from GetNameProxy (MarcoFalke)

Pull request description:

  Currently the getters have a mutable reference as inout param and return a bool to indicate success. This is confusing, because the success bool is redundant with the `IsValid()` state on the proxy object.

  So in theory, the functions could reset the mutable proxy object to an invalid state and return `void`.

  However, this would also be confusing, because devs can forget to check `IsValid()`.

  Fix all issues by using `std::optional<Proxy>`, where devs no longer have to check `IsValid()` manually, or a separate bool. Note that new code in the repo is already using `std::optional<Proxy>`, see `git grep 'std::optional<Proxy>' bitcoin-core/master`. Also, `std::optional<Proxy>` will enforce checking at compile time, whereas calling `Proxy::IsValid` is not enforced.

ACKs for top commit:
  achow101:
    ACK fa73ed467c
  sedited:
    ACK fa73ed467c
  ViniciusCestarii:
    ACK fa73ed467c
  frankomosh:
    Code Review ACK fa73ed467c. Good refactor, correctly replaces the bool + mutable reference output parameter pattern with `std::optional<Proxy>` across `GetProxy` and `GetNameProxy`. Semantics are preserved.

Tree-SHA512: c6a1e1d1691958d2e6507e32e3484f96703fba03ccc710145ae2fb84b1254fb0e6e1d8d78e9b572daf5ea485247b73568704881762379b50bcf939a35494dd13

src/node/interfaces.cpp

@@ -187,7 +187,7 @@ public:
         args().WriteSettingsFile();
     }
     void mapPort(bool enable) override { StartMapPort(enable); }
-    bool getProxy(Network net, Proxy& proxy_info) override { return GetProxy(net, proxy_info); }
+    std::optional<Proxy> getProxy(Network net) override { return GetProxy(net); }
     size_t getNodeCount(ConnectionDirection flags) override
     {
         return m_context->connman ? m_context->connman->GetNodeCount(flags) : 0;