define HTTP request methods at module level outside of class

This is a refactor to prepare for matching the API of HTTPRequest
definitions in both namespaces http_bitcoin and http_libevent. In
particular, to provide a consistent return type for GetRequestMethod()
in both classes.

src/httpserver.h

@@ -45,6 +45,14 @@ struct evhttp_request;
 struct event_base;
 class CService;
 
+enum class HTTPRequestMethod {
+    UNKNOWN,
+    GET,
+    POST,
+    HEAD,
+    PUT
+};
+
 namespace http_libevent {
 class HTTPRequest;
 
@@ -96,14 +104,6 @@ public:
     explicit HTTPRequest(struct evhttp_request* req, const util::SignalInterrupt& interrupt, bool replySent = false);
     ~HTTPRequest();
 
-    enum RequestMethod {
-        UNKNOWN,
-        GET,
-        POST,
-        HEAD,
-        PUT
-    };
-
     /** Get requested URI.
      */
     std::string GetURI() const;
@@ -114,7 +114,7 @@ public:
 
     /** Get request method.
      */
-    RequestMethod GetRequestMethod() const;
+    HTTPRequestMethod GetRequestMethod() const;
 
     /** Get the query parameter value from request uri for a specified key, or std::nullopt if the
      * key is not found.
@@ -287,7 +287,7 @@ class HTTPRemoteClient;
 class HTTPRequest
 {
 public:
-    std::string m_method;
+    HTTPRequestMethod m_method;
     std::string m_target;
     HTTPVersion m_version;
     HTTPHeaders m_headers;