Simplify implementation #355
Description
Related to #321, there are several layered APIs in the implementation which can be simplified/removed.
When the code was originally written it was less clear what the precise shape of things needed to be, so we explored. Now that the world has moved on and the community is settling around the common C2SP tlog-witness and http-bastion specs, we can take the opportunity to tighten the scope of this implementation and simplify/remove things which are no-longer needed/can be done in a simpler way.