proposal: reflect: back out multiple keys in key:value pair in struct tag

[eihigh]

Proposal #40281 was once accepted in Go1.16, but was reverted due to breaking changing found.

Now we have new features to improve compatibility. So it would be time to try again.

cc @scorsi

[scorsi]

Great idea, I still think that's a missing feature !

[ianlancetaylor]

The reflect package would have to know what Go version should be used for the caller for reflect.StructTag.Get. How can that be implemented?