Using a IPv6 literal as the address in a STRICT_DNS cluster generates invalid TCP URLs

Envoy fails to parse a URL generated by resolving the xDS server name to an IPv6 address:

```
[2020-03-23 12:32:58.813][1][debug][init] [source/common/init/watcher_impl.cc:27] init manager Server destroyed
malformed url: tcp://2001:db8:10a:c751::ffff:42ba:8001
```

The URL should be ` tcp://[2001:db8:10a:c751::ffff:42ba]:8001`, which is parsed by `Utility::parseInternetAddressAndPort()`.

It is likely that the caller is using `Utility::portFromTcpUrl()`, which doesn't call the full parser. This seems like a related but distinct problem.

A brief grep looks like there are a few places that could generate URLs of this form:
```
source/common/upstream/logical_dns_cluster.cc: dns_url_ = fmt::format("tcp://{}:{}", socket_address.address(), socket_address.port_value());
source/common/upstream/strict_dns_cluster.cc: fmt::format("tcp://{}:{}", socket_address.address(), socket_address.port_value());
test/common/network/connection_impl_test.cc: fmt::format("tcp://{}:1", Network::Test::getLoopbackAddressUrlString(GetParam()))),
test/extensions/filters/udp/udp_proxy/udp_proxy_integration_test.cc: fmt::format("tcp://{}:{}", Network::Test::getLoopbackAddressUrlString(version_), port));
test/extensions/filters/udp/udp_proxy/udp_proxy_integration_test.cc: fmt::format("tcp://{}:{}", Network::Test::getLoopbackAddressUrlString(version_), port));
test/extensions/filters/udp/udp_proxy/udp_proxy_integration_test.cc: fmt::format("tcp://{}:{}", Network::Test::getLoopbackAddressUrlString(version_), port));
test/integration/http_integration.cc: cluster, fmt::format("tcp://{}:80", Network::Test::getLoopbackAddressUrlString(version_)))};
test/integration/integration.cc: fmt::format("tcp://{}:{}", Network::Test::getLoopbackAddressUrlString(version), port)),
test/integration/integration.cc: fmt::format("tcp://{}:{}", Network::Test::getLoopbackAddressUrlString(version_), port)),
test/integration/utility.cc: fmt::format("tcp://{}:{}", Network::Test::getLoopbackAddressUrlString(ip_version), port));
test/integration/utility.cc: fmt::format("tcp://{}:{}", Network::Test::getLoopbackAddressUrlString(version), port)),
```

*Relevant Links:*

Reported in https://github.com/projectcontour/contour/issues/2354

The contour issue contains the full bootstrap config, but the relevant part is most likely the cluster that points to the xDS server:
```
"static_resources":
{
"clusters":
[
{
"name": "contour",
"alt_stat_name": "projectcontour_contour_8001",
"type": "STRICT_DNS",
"connect_timeout": "5s",
"load_assignment":
{
"cluster_name": "contour",
"endpoints":
[
{
"lb_endpoints":
[
{
"endpoint":
{
"address":
{
"socket_address":
{
"address": "contour",
"port_value": 8001,
},
},
},
},
],
},
],
},
...
```