acme/internal: add a prober program
acme/internal: add a prober program

While working on the RFC 8555 implementation for golang/go#21081,
I've been also manually verifying the functionality against
various CAs both draft-02 and RFC versions.

This does not of course replace automated tests
yet it might be useful to others.

It is not a _test.go file because the program may require
interactive user input for dns-01 challenge types and needs
to flush logs to stderr during its run.

Change-Id: Ia324b08b30308441cc0705648e30c0112b0fa0c8
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/194681
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/crypto,golang/crypto,golang/crypto