Let fits compress their own output CSVs

#1027/#1217 make read_cmdstan_csv() and as_cmdstan_fit() read gzip and bzip2 compressed CSVs, but there's no good way to get a fit object's own files compressed. If you gzip the files yourself, the fitted model object still points at the plain .csv paths and fit$draws() will break. When #1217 is merged, the advice for users is basically "finish with the fit, compress, then rebuild it with as_cmdstan_fit()", which is awkward.

But we could add a compress = c("none", "gzip", "bzip2") argument in two places to make this easier for users:

• The fitting methods ($sample(), $optimize(), etc.), alongside output_dir and output_basename. Once CmdStan finishes, cmdstanr compresses the output files in place with gzfile()/bzfile() and records the new paths, so the fit works as usual with no extra call. I guess latent dynamics files should be compressed too when save_latent_dynamics = TRUE. We can probably skip small files (profile, metric, config).
• $save_output_files() should support compressing after the fact.

This would break anything that gives file paths to a CmdStan binary like $cmdstan_summary(), $cmdstan_diagnose(), generate_quantities(fitted_params = fit), and laplace(mode = fit). We could either error in those cases or we could decompress to a temp file when needed. Or maybe there's another option. Something to think about.

A few other things that aren't ideal, but can just be documented clearly so users are aware:

• CmdStanMCMC reads the sampler diagnostics right after sampling to print warnings, so that first read requires a decompression. We could avoid by compressing after the fit is build (instead of right after run_cmdstan()) but that's a more complicated implementation and I think we should keep it simpler.
• Every read of a compressed file is slower than the plain CSV. Just need to document this so users keep in mind the tradeoff.

I think reading in the compressed files needs gzip/bzip2 on the PATH (from RTools on Windows)

Trace the fitting methods such as $sample() and $optimize(), along with $save_output_files(), and review how read_cmdstan_csv() and as_cmdstan_fit() store and consume paths. Define how the compress option should update fit-owned files, including latent dynamics, and how commands such as $cmdstan_summary(), $cmdstan_diagnose(), generate_quantities(), and laplace() handle compressed paths. Done should include working gzip and bzip2 output plus clear handling or documentation for affected commands and performance requirements.