Add problem details JSON body with supported content type

[arthurcro]

Motivation

Closes apple/swift-openapi-generator#963

When returning a 406, right now we only return the code. Figuring out what code (and thus content type) we were actually trying to return requires the user to dig into the OpenAPI doc/server codebase more:

swift-openapi-runtime/Sources/OpenAPIRuntime/Errors/RuntimeError.swift

Line 168 in 49689c7

case .unexpectedAcceptHeader: .notAcceptable

Modifications

Adds the error returned as the http body of the error response structued as a product details https://www.rfc-editor.org/rfc/rfc9457.html.

Along with human readable informations, it adds two extra fields expected and received which contains machine readable information helping to figure out which accept header was received and which were expected.

Result

{
  "details":"Unexpected Accept header: text\/plain",
  "expected":"application\/json",
  "received":"text\/plain",
  "status":406,
  "title":"Unexpected Accept header",
  "type":"about:blank"
}

Test Plan

Added testRuntimeError_withUnexpectedAcceptHeader_returnsProblemDetails to Test_RuntimeError.