addrman: add function to return size by network and table

For now, the new functionality will be used in the context of
querying fixed seeds. Other possible applications for
future changes is the use in the context of making automatic
connections to specific networks, or making more detailed info
about addrman accessible via rpc.

src/addrman.h

@@ -102,6 +102,15 @@ public:
     //! Return the number of (unique) addresses in all tables.
     size_t size() const;
 
+    /**
+    * Return size information about addrman.
+    *
+    * @param[in] net              Select addresses only from specified network (nullopt = all)
+    * @param[in] in_new           Select addresses only from one table (true = new, false = tried, nullopt = both)
+    * @return                     Number of unique addresses that match specified options.
+    */
+    size_t Size(std::optional<Network> net, std::optional<bool> in_new) const;
+
     /**
      * Attempt to add one or more addresses to addrman's new table.
      *