bake: multi ips support for extra hosts

[crazy-max]

follow-up

• bake: extra-hosts support #3234
• add support for extra_hosts building with bake compose#12935

Adds multiple IPs support for extra hosts value using , separator for Bake.

target "default" {
  extra-hosts = {
    myhost = "1.2.3.4"
    myhostmulti = "162.242.195.81,162.242.195.82"
  }
}

FROM busybox
RUN cat /etc/hosts

$ docker buildx bake
...
#7 [2/5] RUN cat /etc/hosts
#7 0.243 127.0.0.1      localhost buildkitsandbox
#7 0.243 ::1    localhost ip6-localhost ip6-loopback
#7 0.243 1.2.3.4        myhost
#7 0.243 162.242.195.81 myhostmulti
#7 0.243 162.242.195.82 myhostmulti
#7 DONE 0.3s