Well-known attributes (Kuadrant RFC-0002)

[guicassolato]

RFE: Implementation of Kuadrant's Well-known attributes (RFC-0002).

Possible steps for the implementation

• Define a struct that models all applicable Well-known attributes, except auth.*
• Implement a NewWellknownAttributes function that instantiates the struct from an Envoy ext-authz CheckRequest input
• Define a proper AuthorizationJson struct that merges the new Well-known attributes type into it, with json:"" annotation
• Implement a NewAuthorizationJson function that instantiates the struct from an Envoy ext-authz CheckRequest and AuthPipeline input
• Deprecate the context.* attributes from the Authorization JSON (for future removal)