Include explicit stan version in reference posterior computations

Now only rstan version is included in the reference posterior. Since pystan might not follow stan versions, we should add an additional slot for stan version in all reference posterior computations.

Current structure:

    "rstan_version": "rstan 2.21.1",
    "r_Makevars": ... 
    "r_version": ...
    "r_session":  ...
    "r_summary_statistic": "posterior R package, version 0.1.2"

Proposed structure:

    "stan_version": "2.21",
    "interface_version": "rstan 2.21.1",
    "r_Makevars": ... 
    "r_version": ...
    "r_session":  ...
    "r_summary_statistic": "posterior R package, version 0.1.2"

Functionality to use:
https://mc-stan.org/rstan/reference/stan_version.html

Start by locating the code that builds metadata for reference posterior computations and read how rstan_version is currently populated. Use the linked stan_version functionality to add the separate Stan version and interface version fields, then verify that all reference posterior computations expose the proposed metadata structure.