feat: Add a "namespace" parameter to the "plugin_config" of apisixRoute, to share the same apisixPluginConfig between all namespaces of apisixRoutes

[seethedoor]

Description

We are using apisixRoutes, and now we need to add a common plugin_config to all of the routes. But I found that apisixRoute spec params only support plugin_config_name, and it will only search apisixPluginConfig in the same namespace of the apisixRoute.
This will require to create the same apisixPluginConfigs for all namespaces and maintain these apisixPluginConfigs as namespaces change. If we can add a plugin_config_namespace parameter under the apisixRoute spec, combined with the plugin_config_name, we can solve the above problem with just one apisixPluginConfig.

[Gallardot]

Good idea. I believe this will help to reduce maintenance costs.

[Revolyssup]

@seethedoor Thanks for reporting. I'll take this up