Support flag query parameters

[adamw]

Currently we don't support flag-like query parameters.

query[Option[String]]("flag") decodes ?flag into a None and ?flag= into a Some("")

I'm not sure how this should be represented in an OpenAPI spec. There is the allowEmptyValues field (see https://swagger.io/specification/), but it's deprecated and shouldn't be used. There is a large discussion here: OAI/OpenAPI-Specification#1573, but I think it doesn't reach a conclusion as to how to represent this in OpenAPI. So more research would be needed.

On the tapir side, I think we'll need to add a new field to Query which would store the value, that should be used for the query parameter, in case no value is provided. Sth like query[String]("flag").valueWhenNoValue("true"), except with a better name :)

[eikek]

For the time being a possible workaround would be to combine two definitions:

def queryFlag(name: String, description: String = "") = {
  // this is false when the query parameter exists without a value
  val a = query[Option[Boolean]](name)
    .description(description)
    .example(EndpointIO.Example(true.some, "true".some, None))
    .example(EndpointIO.Example(false.some, "false".some, None))

  // this is true when the query parameter exists without a value, but same as (a) otherwise
  // but it is not added to the documentation
  val b = queryParams
    .map(qp => qp.getMulti(name).exists(_.forall(_.equalsIgnoreCase("true"))))(flag =>
      QueryParams.fromMap(Map(name -> flag.toString))
    )

  // combining both does the job…
  (a / b).map(_._2)(flag => (flag.some, flag))
}