change: don't expose public API by default & remove plugin interceptor

[bzp2010]

What this PR does / why we need it:

Tweak the public-api plugin to no longer expose the public API by default, while removing the skip global rules feature and configuration.

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[bzp2010]

Review guide

There is a lot of content in this PR revision, so I am providing a modifications list here.

Modified content

1. Core modifications: apisix/admin/plugin_metadata.lua, apisix/api_router.lua, apisix/init.lua
   Remove the public API match in the access phase and remove the plugin interceptor.

2. Plugin modification: apisix/plugins/public-api.lua
   Remove the skip global rule, it will now be executed unconditionally (since the public API is now also managed by the normal HTTP request router)

3. Tests based on Test::Nginx: Several files in *.t
   Add a pre-step for creating public API routes (those provide public API).
   Modify the default settings for partial GET /t and no_error_log.

4. CLI tests: t/cli/test_admin.sh, t/cli/test_prometheus.sh
   Add a pre-step for creating public API routes (node-status and prometheus plugin)

5. Chaos tests
   Add a pre-step for creating public API routes (prometheus plugin)

Other

This PR does not contain documentation for the public API and changes to its documentation involving plugin interceptors and plugin development.