const correctness for services

[bob-carpenter]

Summary:

Two arguments to the service functions (all functions in stan/services/...) should be made constant:

• io::var_context: this should be easy as var_context has appropriate const declarations
• Model (templated); this will require a lot of fixes as the generated code for our models does not have appropriate const correctness.

It would be nice to have both, but a PR to solve either one on its own would be welcome.

Current Version:

v2.19.1