Custom algorithms specifications as public API

We should make the API to define algorithm specifications public. This requires:

• Adding the algorithm to use to the public API
• Document the format of the algorithm definition
• Add a what's new entry (!95 (comment 80285))

The following discussion from !95 (merged) should be addressed:

• @anton-akhmerov started a discussion:

  In a follow-up MR we should make the algorithm a part of the interface.