[Post-#706] Improve TOML config file UX

[samuelburnham]

Doc comment for CliSettings:

// NOTE: Config settings share the same file for both the Lurk library and the CLI.
// It's good practice to avoid duplication of shared settings like public_params_dir
// in downstream configs like these to prevent conflicts.

From #706 (comment):

This is a bit confusing, as a user-specified rc and limit value in the config file will affect the CLI but not the Lurk library or benchmarks as one might expect. It may be more user-friendly to have a separate cli.toml rather than sharing settings with lurk.toml, or better yet support a table-based TOML structure like the following:

[lurk]
public_params_dir = "/path/to/public_params"

[cli]
proofs_dir = "/path/to/proofs"
rc = 100
limit = 100000000