Allow setting multiple header values in KongRoute's spec

[czeslavo]

Problem Statement

KongRoute's spec.headers field was initially defined as map[string]string while the real type expected by the Konnect API is map[string][]string. The goal of this issue is to allow setting multiple values of the same header.

Proposed Solution

As the API is already public, we should not break the schema. Because of that, adding a new headerValues (name to be discussed) field is proposed, deprecating the old headers at the same time.

Acceptance Criteria

• It is possible to set multiple values for a single header key in the KongRoute's spec