unify json config loading

[martyall]

This PR stacks on #16163

The Problem

There should only be one way to load json files to construct a value of type Runtime_config.t, we have several:

• Mina_cli_entrypoint checks system directories for an installed config file, other places do not
• Mina_cli_entrypoint logs verbosely where it finds certain values that belong the daemon object. This requires special handling to load these values only to gain a debug log -- we don't do this for any other fields or in any other CLI context, and would require more complicated handling. The fact that the ~keyname arguments for the logger are not derived from the value means they can drift.
• Mina_cli_entrypoint will also attempt to write configuration as it is reading it. This addition was made something like 4 years ago. Rather than attempting to modify configuration on a users file system, at this point it seems wiser to just throw a parse exception.

The solution

• Introduce a module Runtime_config.Json_loader to handle all Runtime_config loading logic, use this in all executables.
• Use the same standard for loading daemon field values as we do for everything else
• Remove the old update_config function that writes a json as it is loading.

[martyall]

!ci-build-me

[martyall]

!ci-nightly-me

[martyall]

Nightly tests here https://buildkite.com/o-1-labs-2/mina-o-1-labs/builds/35530#_

Passes everything except for merge cleanly checks, which will wait to review and supporting PRs are merged