`info` method - expose information about the model

Summary:

Add a new info method that lets tools (or users) get info about the model to help e.g. deciding if the model needs recompilation.

Description:

Currently, a lot of the metadata about a compiled model can only be retrieved by running inference (Stan version, stanc flags) or not at all (cpp compile options, compiler, ...). This limits the ability of wrappers like cmdstanr to make good decisions on when to recompile a model as a change in compiler options or Stan version will not be recognized by checking modification times.

Beyond choosing to recompile or not, other tools may benefit from this metadata in my own work, the SBC package tries to do caching of its results and needs to check if the model it is given is substantially equivalent to a model the cached results were compiled with. It could rely on modification time, but this could result in some unnecessary recomputations (e.g. when the model is modified, found to be problematic and then the change is reverted). For those more extended usages by tools, it might make sense to also include the Stan code and the contents of user-provided extra .hpp in the information stored.

Since all of the information is known at a compile time, a simple implementation would have a .json (or other format) created at compile time and embedded as a resource. The output format could be configurable, but since the primary consumers are likely to be tools, it should probably default to JSON or similar.

Additional information:

If there is an agreement on implementing this, I'd be happy to write a PR.

Current Version:

v2.32.2

The issue names no files, tests, or entry points; begin by locating where compiled model metadata and command methods are handled. Define the metadata exposed by the new info method, including compilation details and possibly Stan or header contents, then determine how compile-time JSON or another embedded format should be produced and verify the output supports recompilation and caching decisions.