Merge bitcoin/bitcoin#35829: http: Make class fields private and make HTTPResponse a struct

5e0d7a2 refactor: Drastically narrow scope of http_bitcoin namespace and rename it to bitcoin_http (Hodlinator)
8f9fd86 refactor: Make HTTPRemoteClient fields private (Hodlinator)
d72f67f refactor: Expose additional HTTPRemoteClient fields through accessors (Hodlinator)
10bbae3 refactor: Expose HTTPRemoteClient fields to tests through methods (Hodlinator)
5b06d90 refactor: Replace HTTPServer::MaybeDispatchRequestsFromClient() with HTTPRemoteClient::TryReadRequest() (Hodlinator)
a1183c0 refactor: Extract Send() and Receive() into HTTPRemoteClient from HTTPServer (Hodlinator)
6d9b61d refactor: Extract HTTPRemoteClient::MaybeDisconnect() from HTTPServer::DisconnectClients() (Hodlinator)
6fec8d6 refactor: Make HTTPRequest fields private (Hodlinator)
b8cd772 refactor: Make HTTPRequest::GetHeader() return saner optional type (Hodlinator)
e5be0dc refactor: Make HTTPResponse a struct since all fields are public (Hodlinator)

Pull request description:

  The new HTTP server implementation in v32 has `HTTPServer` reaching into and modifying fields of `HTTPRemoteClient` and `HTTPRequest`. This PR encapsulates field data of the latter 2 types which enforces invariants and reduces cognitive load[^1]. Exposing data through accessor methods also implies adding lock annotations.

  Commits:
  * Makes `HTTPResponse` a struct since it is used that way. (bitcoin/bitcoin#35182 (comment)) [^2]
  * `HTTPRequest`:
    * Saner return type for `GetHeader()` (old type was mirroring the now removed libevent-wrapper and made later commits ugly).
    * Make fields private.
  * Simplifies boolean logic in `HTTPServer::DisconnectClients()`. (bitcoin/bitcoin#35182 (comment))
  * Extraction of `HTTPServer` functions into `HTTPRemoteClient`:
    Refactors `HTTPRemoteClient` to be more self-contained rather than having `HTTPServer` reach into the fields of other objects. (bitcoin/bitcoin#35182 (comment), bitcoin/bitcoin#35182 (comment))
  * Severely narrows `http_bitcoin` namespace and renames it to `bitcoin_http` (bitcoin/bitcoin#35182 (comment))

  Follow-up to #35182.

  [^1]: Core Guidelines: C.9: Minimize exposure of members - https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c9-minimize-exposure-of-members
  [^2]: Core Guidelines: C.2: Use class if the class has an invariant; use struct if the data members can vary independently - https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c2-use-class-if-the-class-has-an-invariant-use-struct-if-the-data-members-can-vary-independently

ACKs for top commit:
  achow101:
    ACK 5e0d7a2
  janb84:
    ACK 5e0d7a2
  winterrdog:
    tACK 5e0d7a2

Tree-SHA512: e1c5aa067538e31247ca74923e451038c90750ccc941ae16711dd976c8cd750bd1afaee6e4378aeee91440f7727955d9bfb32aa25a0a745613d0d771a674ebc8

Author: fanquake

src/httprpc.cpp

@@ -26,7 +26,6 @@
 #include <string>
 #include <vector>
 
-using http_bitcoin::HTTPRequest;
 using util::SplitString;
 using util::TrimStringView;
 
@@ -202,8 +201,8 @@ static void HTTPReq_JSONRPC(const std::any& context, HTTPRequest* req)
         return;
     }
     // Check authorization
-    std::pair<bool, std::string> authHeader = req->GetHeader("authorization");
-    if (!authHeader.first) {
+    std::optional<std::string> auth_header = req->GetHeader("authorization");
+    if (!auth_header) {
         req->WriteHeader("WWW-Authenticate", WWW_AUTH_HEADER_DATA);
         req->WriteReply(HTTP_UNAUTHORIZED);
         return;
@@ -213,7 +212,7 @@ static void HTTPReq_JSONRPC(const std::any& context, HTTPRequest* req)
     jreq.context = context;
     jreq.peerAddr = req->GetPeer().ToStringAddrPort();
     jreq.URI = req->GetURI();
-    if (!RPCAuthorized(authHeader.second, jreq.authUser)) {
+    if (!RPCAuthorized(*auth_header, jreq.authUser)) {
         LogWarning("ThreadRPCServer incorrect password attempt from %s", jreq.peerAddr);
 
         /* Deter brute-forcing