Issue description

Current situation

At the moment, you add a plugin in the request flow by referencing with its name and setting the configuration. The plugin is effectively a singleton.

This approach has some downsides:

• You can use a plugin only once in the request flow. Users cannot add the same plugin multiple times with different configurations.
• For maintainers, it means that they need to duplicate code and reference it with different names and priorities. For example, the ext-plugin-pre-req and the ext-plugin-post-req do the same - run arbitrary Lua code.
• Users cannot change the priority of a plugin, it's hard-coded (see feat: As a User, I want to be able to configure the priority of each plugin so I don't need to change the source code #6504)
• Plugin developers need to handle every possible configuration case (see request help: How to current limit both in minutes and day by using plugin limit-count #6197)

Proposal

In Object-Oriented speak, make Plugin a class and introduce PluginInstance "objects" (or something similarly named):

• Plugin defines the code
• PluginInstance references the Plugin and associates it with a priority and a configuration

Decoupling the code from the configuration/priority would bring much more freedom for users.