Investigate header percent-encoding with clarifications in recent OpenAPI versions

The question of percent-encoding header values over the evolution of the OpenAPI specification has been a bit messy, and 3.1.2 adds to it again: https://spec.openapis.org/oas/v3.1.2.html#appendix-d-serializing-headers-and-cookies

This issue tracks tracking down the correct behavior, and possibly defaulting to different behavior based on the version of the OpenAPI document - to avoid making backwards-incompatible changes.

This might require extending OpenAPI Runtime to allow customizing whether to percent-encode header values or not (maybe can even be added to the Configuration struct), and try to use the correct behavior on the newer version of OpenAPI docs where this behavior was clarified.

Start with OpenAPI Runtime and the Configuration struct, then compare their current header serialization behavior with Appendix D of the OpenAPI 3.1.2 specification. Done means the runtime can apply the correct percent-encoding behavior for the relevant OpenAPI document version without introducing a backwards-incompatible default.

Written by the indexing model from the issue text.