[rpc/net] Introduce addconnection to test outbounds & blockrelay

Add a new RPC endpoint to enable opening outbound connections from
the tests. The functional test framework currently uses the addnode RPC, which
has different behavior than general outbound peers. These changes enable
creating both full-relay and block-relay-only connections. The new RPC
endpoint calls through to a newly introduced AddConnection method on
CConnman that ensures we stay within the allocated max.

src/net.h

@@ -955,6 +955,19 @@ public:
     bool RemoveAddedNode(const std::string& node);
     std::vector<AddedNodeInfo> GetAddedNodeInfo();
 
+    /**
+     * Attempts to open a connection. Currently only used from tests.
+     *
+     * @param[in]   address     Address of node to try connecting to
+     * @param[in]   conn_type   ConnectionType::OUTBOUND or ConnectionType::BLOCK_RELAY
+     * @return      bool        Returns false if there are no available
+     *                          slots for this connection:
+     *                          - conn_type not a supported ConnectionType
+     *                          - Max total outbound connection capacity filled
+     *                          - Max connection capacity for type is filled
+     */
+    bool AddConnection(const std::string& address, ConnectionType conn_type);
+
     size_t GetNodeCount(NumConnections num);
     void GetNodeStats(std::vector<CNodeStats>& vstats);
     bool DisconnectNode(const std::string& node);