Embed original interface into Mock struct

[arxeiss]

Requested feature

I would like to have option to embed original interface into generated Go structure.

Why the feature is needed

If the interface has unexported methods, the generated mock cannot be used as it does not fulfill the original interface. Embedding it however solves the issue. As discussed here https://groups.google.com/g/golang-nuts/c/6hpUErAfMHI

(Optional) Proposed solution

Solution should be pretty easy, as @stoikheia created PR in old repo golang/mock#683

[stoikheia]

@arxeiss
Thanks for open this issue with introducing my PR.

Your problem will be resolved if that suggestion is applied.
I'll port it to this repo as a PR this week end.

[ash2k]

Just encountered this issue. Trying to generate mocks for OpenTelemetry interfaces and they have embedded interfaces with private methods. Sigh.

Example: https://github.com/open-telemetry/opentelemetry-go/blob/98b32a6c3a87fbee5d34c063b9096f416b250897/metric/asyncint64.go#L42-L49

[lance6716]

@stoikheia kindly ping. Now I meet this problem. And I also ported your PR but need you to sign CLA