Make dashboard usable for readonly inspection in (API) standalone mode

# Feature request

## Please describe your feature

I'd like to be able to inspect an apisix instance's current configuration through the dashboard.
As discussed in #3257, when configuring apisix to run in standalone mode, the dashboard does not work, as the API returns 404 for most (all?) resources.
This is problematic when troubleshooting issuses, e.g. when a route does not behave as expected by configuration (through K8s CRDs).

## Describe the solution you'd like

Having a working dashboard even in standalone mode.

## Describe alternatives you've considered

Get the configuration dumped via a CLI tool or API. (However, this is less approachable for new users.)

## Additional context

#3257

Start by reading the discussion in #3257 and tracing the dashboard API requests that return 404 in standalone mode. Determine which configuration resources can be inspected without write operations; done means the dashboard provides readonly inspection of the current standalone configuration and no longer fails on those resources.

Written by the indexing model from the issue text.