Draft: [whyconf] lookup for external prover config

The goal of this MR is to provide low-level access to supported provers, even for non-installed ones, in order to prepare tasks to be sent over the network towards a prover server.

Two functions are added to auto-detection API: the ability to list all supported prover-version-altern, and the ability to obtain a prover-config for one specific prover-version-altern. This prover config is only usable for preparing tasks, it is not suitable for running provers, since there is no command-line associated with it, which is now checked before executing a prover.

Remark: the why3 tools are not affected since they never build prover-config this way.