mgmt: hawkbit: interface support for ip addresses and domain name

Previously, hawkbit interface only supported a url/hostname and a port,
and internally it resolves to an IP address. This does not work for
network layers that rely on NAT64, like OpenThread.  Zephyr's
implementation of `getaddrinfo` is not aware of NAT64.  DNS will resolve
an IPV4 address that needs to be converted to IPV6 with the NAT64
prefix.

This commit alters the Hawkbit interface to allow providing an explicit
domain name as a string via `server_domain`, and an already resolved IP
address as `server_addr`.

This commit changes the usage of `hawkbit_runtime_config.server_addr` to
point to either an IP address or domain name. It adds a new Kconfig
(`HAWKBIT_USE_DOMAIN_NAME`) to specify an explicit domain name and adds
a new variable `hawkbit_runtime_config.server_domain`. If
`HAWKBIT_USE_DOMAIN_NAME` is enabled and a user provides an IP address
to `server_addr`, the user must provide a domain name to
`server_domain`.

Signed-off-by: Neal Jackson <neal@blueirislabs.com>