Refactoring for external auth. Vol I

[didierofrivia]

Part of the work needed for https://github.com/Kuadrant/wasm-shim/issues/58

This PR is the first of hopefully not too many that aims to re-arrange and open the code in order to implement External Auth.

Notes:

• Renames RateLimitPolicy struct to Policy
• Keeps config with rateLimitPolicies field, but renames it to policies field within the struct PluginConfiguration when deserializing.
• Delegates the building of descriptors to the Policy instead of Filter
• Missing configuration change, TODO alongside a PR to Kuadrant Operator. Its still functional since it's serde::renaming the field