cli gen: improve flag set defaults from message literals

[kralicky]

This improves how field values from nested message literals are rendered as strings for flag set defaults.

---

For example, given:

message MetricsCapabilitySpec {
  .config.v1beta1.RulesSpec rules = 1 [(cli.flag_set).default = {
    [type.googleapis.com/config.v1beta1.RulesSpec]: {
      discovery: {
        prometheusRules: {
          searchNamespaces: [""]; // search all namespaces
        }
      }
    }
  }];
}

The default field above will translate into this call to flagutil.SetDefValue:

func (in *MetricsCapabilitySpec) FlagSet(prefix ...string) *pflag.FlagSet {
	// ...

	fs.AddFlagSet(in.Rules.FlagSet(append(prefix, "rules")...))
	flagutil.SetDefValue(fs, strings.Join(append(prefix, "rules", "discovery.prometheus-rules.search-namespaces"), "."), `[""]`)

	// ...
}

Partial fix for #1735

[alexandreLamarre]

LGTM, but given the complexity of generated code and the multitude of edge cases it could present, there should be unit tests for this package

[kralicky]

As much as I agree, unit tests for this code would be very time consuming and it would probably be more efficient to instead write CLI tests that use the generated code, which we need to do anyway