[5535] RelayInfo parsing handles both ip-address and ip-addresses

[5535] RelayInfo parsing handles both ip-address and ip-addresses

src/lib/dhcpsrv/parsers/dhcp_parsers.*
    RelayInfoParser::parse() - reworked to support either
    ip-address or ip-addresses

    RelayInfoParser::addAddress() - new parser helper method

src/lib/dhcpsrv/parsers/shared_network_parser.cc
    SharedNetwork4Parser::parse()
    SharedNetwork6Parser::parse()
    - both now parse "relay" element (was missing)

src/lib/dhcpsrv/tests/shared_network_parser_unittest.cc
    Modified to support testing "relay" element parsing
    Added new tests:
        TEST_F(SharedNetwork4ParserTest, relayInfoTests)
        TEST_F(SharedNetwork6ParserTest, relayInfoTests)