Add flatten fields flag support

[JulianToledano]

     Can we maybe add flag support in a follow-up?

So that this can work for instance:

PositionalArgs: []*autocliv1.PositionalArgDescriptor{
	{ProtoField: "grantee"},
	{ProtoField: "permissions.limit_type_urls"},
},
FlagOptions: map[string]*autocliv1.FlagOptions{
	"permissions.level": {Name: "level", DefaultValue: "some-msgs"},
},

Originally posted by @julienrbrt in #22890 (review)