Allow name@domain.tld for OpenAlias lookups

Based on tewinget's update.
Make OpenAlias address format independent of existing DNS functions.

Add tests.

Test:

make debug-test
cd build/debug/tests/unit_tests
# test that regular DNS functions work, including IPv4 lookups.
# also test function that converts OpenAlias address format
make && ./unit_tests --gtest_filter=DNSResolver*
# test that OpenAlias addresses like donate@getmonero.org work from
# wallet tools
make && ./unit_tests --gtest_filter=AddressFromURL.Success

src/common/dns_utils.h

@@ -100,6 +100,18 @@ public:
   // TODO: modify this to accomodate DNSSEC
    std::vector<std::string> get_txt_record(const std::string& url, bool& dnssec_available, bool& dnssec_valid);
 
+  /**
+   * @brief Gets a DNS address from OpenAlias format
+   *
+   * If the address looks good, but contains one @ symbol, replace that with a .
+   * e.g. donate@getmonero.org becomes donate.getmonero.org
+   *
+   * @param oa_addr  OpenAlias address
+   *
+   * @return dns_addr  DNS address
+   */
+  std::string get_dns_format_from_oa_address(const std::string& oa_addr);
+
   /**
    * @brief Gets the singleton instance of DNSResolver
    *