Previously the `--kubernetes-host_aliases` flag would be ignored. The
CLI flag module uses Go Reflection to recursively generate
command-line flags for the `RunnerConfig` structure. However, the flag
parser inside `golang-cli-helpers` does not attempt to unmarshal a
slice of `KubernetesHostAliases` structs because it does not know how
to map a string to that struct.

As documented in https://github.com/jessevdk/go-flags, a struct can
define an `UnmarshalFlag` method.  We can define this method to parse
JSON data from the command-line.

Using Go Reflection, I also determined that this was the only
command-line argument that needed this custom parsing. I omitted that
test case to keep the review straightforward, but I can add it in a
separate commit.

Relates to https://gitlab.com/gitlab-org/gitlab-runner/-/issues/37802

Changelog: changed