fix(plugins): load wasm filter plugins before external plugins

[flrgh]

Summary

In 4059a31 (#13843) we added a plugin-like interface for Wasm filters.

We now have 3 sources for plugins: Lua, External, and Wasm Filters. When a plugin is enabled or configured, the plugin system follows a resolution order for looking up the plugin handler and schema:

1. Lua => require kong.plugins.<name>.{handler,schema}
2. External => kong.runloop.plugin_servers.load_{handler,schema}(<name>)
3. Wasm Filters => kong.runloop.wasm.plugins.load_{handler,schema}(<name>)

When a user configures Kong with a "bad" entry in pluginserver_names (read: a plugin server that is not actually installed), step #2 of the plugin resolution process throws an exception, because the external plugin subsystem attempts to query a plugin server that does not exist. Importantly, this exception is not encountered when the user has only configured/enabled Lua plugins, because we never reach beyond step #1 of the plugin resolution process.

A side effect of adding the Wasm filter plugin interface is that discovered Wasm filters are added to the global plugins table (kong.configuration.loaded_plugins) when Wasm is enabled. This means that, if Wasm is enabled, and any Wasm filters are installed, we always step through step #2 of the plugin resolution process, triggering an exception if the user has any badly-configured plugin server.

A future change will likely render this scenario unreachable by performing deeper validation of pluginserver_names at startup. For now, a simple fix is just to change the resolution order such that Wasm filters are loaded before we query the external plugin subsystem:

1. Lua
2. Wasm Filters
3. External

Checklist

• The Pull Request has tests
• A changelog file has been created (this fixes an unreleased bug)
• There is a user-facing docs PR

Issue reference

KAG-5911

[flrgh]

note: I removed the cherry-pick label in favor of manually cherry-picking this into the in-progress branch in EE land:

https://github.com/Kong/kong-ee/pull/10742

[flrgh]

I'm going to wait to merge this until after the EE PR is green just to ensure there are no other fixes needed here.